Objective To investigate the intervention effect and mechanism of bone marrow mesenchymal stem cells (BMSCs) on phosphorus metabolism disorder in chronic kidney disease (CKD) rats with 5/6 nephrectomy.Methods The CKD rat model was established by 5/6 nephrectomy. Rats were randomly divided into control group, 5/6 nephrectomy group and BMSCs group, with 6 rats in each group. The control group underwent laparotomy and renal capsule stripping with intact bilateral kidneys preserved; the 5/6 nephrectomy group received 5/6 nephrectomy plus tail vein injection of equal volume of phosphate-buffered saline (PBS); the BMSCs group received 5/6 nephrectomy plus tail vein injection of BMSCs. Renal function and proteinuria (serum creatinine, urea nitrogen, 24-hour urinary protein quantification), serum albumin, mineral metabolism indexes (calcium, phosphorus), and blood lipid profile (total cholesterol, triglyceride, low-density lipoprotein cholesterol and high-density lipoprotein cholesterol) were measured in each group. Serum levels of parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) were detected by enzyme-linked immunosorbent assay (ELISA). Renal pathological changes were observed by HE staining, Masson staining and transmission electron microscopy. The protein expressions of Klotho and FGF23 in rat renal tissues were detected by Western blotting. The mRNA expressions of FGF23, Klotho, FGFR1, FGFR4, NaPi-2a and NaPi-2c in rat renal tissues were detected by quantitative real-time polymerase chain reaction (qRT-PCR). Klotho promoter activity was detected by dual-luciferase reporter assay. Three groups were set up: GV238-Klotho-p-Luc + pRL-TK group, transforming growth factor-β1 (TGF-β1) group and BMSCs treatment group. After 24 h of intervention via Transwell non-contact co-culture system, the ratio of firefly to renilla luciferase was detected to evaluate transcriptional activity.Results Compared with the control group, the levels of 24-hour urinary protein, serum creatinine and urea nitrogen were increased (P < 0.05) and albumin level was decreased (P < 0.05) in the 5/6 nephrectomy group. Compared with the 5/6 nephrectomy group, the levels of 24-hour urinary protein, serum creatinine and urea nitrogen were decreased (P < 0.05) and albumin level was increased (P < 0.05) in the BMSCs group. Compared with the control group, the levels of serum phosphorus, total cholesterol, triglyceride, low-density lipoprotein cholesterol and high-density lipoprotein cholesterol were increased (P < 0.05) in the 5/6 nephrectomy group. Compared with the 5/6 nephrectomy group, the levels of serum phosphorus, total cholesterol, triglyceride, low-density lipoprotein cholesterol and high-density lipoprotein cholesterol were all decreased (P < 0.05) in the BMSCs group. Compared with the control group, serum levels of FGF23 and PTH were increased (P < 0.05) in the 5/6 nephrectomy group. Compared with the 5/6 nephrectomy group, serum levels of FGF23 and PTH were decreased (P < 0.05) in the BMSCs group. Pathological results showed that renal tubular injury, interstitial fibrosis, ultrastructural damage and increased foot process fusion occurred in the 5/6 nephrectomy group, and the above pathological changes were alleviated after BMSCs intervention. Compared with the control group, the relative protein expression of FGF23 was increased (P < 0.05) and the relative protein expression of Klotho was decreased (P < 0.05) in the 5/6 nephrectomy group. Compared with the 5/6 nephrectomy group, the relative protein expression of FGF23 was decreased (P < 0.05) and the relative protein expression of Klotho was increased (P < 0.05) in the BMSCs group. Compared with the control group, the relative mRNA expressions of FGF23, FGFR1, FGFR4, NaPi-2a and NaPi-2c were increased (P < 0.05) and the relative mRNA expression of Klotho was decreased (P < 0.05) in the 5/6 nephrectomy group. Compared with the 5/6 nephrectomy group, the relative mRNA expressions of FGF23, FGFR1, FGFR4, NaPi-2a and NaPi-2c were decreased (P < 0.05) and the relative mRNA expression of Klotho was increased (P < 0.05) in the BMSCs group. Compared with the GV238-Klotho-p-Luc + pRL-TK group, the relative transcriptional activity of Klotho promoter and the relative protein expression of Klotho were decreased (P < 0.05) in the TGF-β1 group. Compared with the TGF-β1 group, the relative transcriptional activity of Klotho promoter and the relative protein expression of Klotho were increased (P < 0.05) in the BMSCs treatment group. BMSCs could enhance Klotho promoter activity and promote its expression.Conclusion BMSCs can improve CKD-related phosphorus metabolism disorder by regulating Klotho expression, providing new evidence for stem cell therapy of CKD.