Objective To investigate the relationship between serum magnesium level and pulmonary function in children with mild asthma.Methods A total of 320 children with mild asthma were selected as the study object. All the children were tested for lung function and serum magnesium level after admission. Symptom control was performed with the child asthma control test questionnaire (ACT). According to the ACT score, the children were divided into well controlled group, partial control group, and uncontrolled group. The differences of pulmonary function and serum magnesium level in children with different symptom control levels were analyzed and compared.Results The total serum magnesium level range was 1.5 to 2.5 mg/dL, and the prevalence of hypomagnesemia was 5.31% (17/320). The ratio of forced expiratory volume in forced vital capacity (FEV₁ / FVC), predicted value of FEV₁%, peak expiratory flow rate (PEFR), and act score in the first second in different asthma control groups were significantly different (P < 0.05), and those in the well controlled group were higher than those in partial control groups and uncontrolled groups (P < 0.05). Multivariate logistic regression analysis showed whether it was left behind children [O^R = 2.065 (95% CI: 1.119, 3.974) ], whether it had family history [O^R = 2.317 (95% CI: 1.214, 4.482) ], duration of disease [O^R = 2.353 (95% CI: 1.329, 5.215) ], treatment time [O^R = 2.147 (95% CI: 1.032, 4.978) ], whether it was complicated with allergic rhinitis [O^R = 2.587 (95% CI: 0.214, 3.365) ], and treatment compliance [O^R = 1.934 (95% CI: 1.021, 3.368) ] were independent risk factors affecting the control level of children with asthma (P < 0.05). Spearman correlation analysis showed that ACT score was positively correlated with FEV₁ and PEFR (r_s = 0.399 and 0.246, all P < 0.05). Serum magnesium level was not correlated with ACT score, FEV₁, and PEFR (P > 0.05), but negatively correlated with treatment time (r_s = -0.162, P < 0.05).Conclusion There is a certain incidence of hypomagnesemia in mild persistent children, but there is no significant correlation between serum magnesium level and asthma symptom control.