Objection To explore the effect of neutrophil elastase (NE) on the diagnosis and clinical outcome of ventilator associated pulmonary infection (VAP). Methods The clinical data of 228 patients undergoing mechanical ventilation in the ICU of our hospital from January 2012 to January 2017 were retrospectively analyzed. The patients were divided into VAP group (n = 86) and non VAP group (n = 142). General data of the two groups, concentration of NE in serum and BALF at different time points, relationship between the concentration of NE in serum and BALF and mechanical ventilation time, CPIS, oxygenation index, mortality and the value of NE in diagnosis of VAP were observed. Results At different time points, there was difference in concentration of NE in serum and BALF among patients (F = 13.495 and 21.832, all P = 0.000). There was difference in concentration of NE in serum and BALF at different time points between VAP group and non VAP group (F = 25.426 and 19.426, all P = 0.000); the concentration of NE in serum and BALF of the VAP group were higher than that of the non VAP group at different time points (P < 0.05). There was difference in variation tendency of NE in serum and BALF between VAP group and non VAP group (F = 14.372 and 9.387, all P = 0.000); the concentration of NE in serum was positively correlated with the duration of mechanical ventilation, CPIS and mortality (rs = 0.753, 0.475 and 0.801, P = 0.000, 0.002 and 0.000), while was negatively correlated with oxygenation index (rs = -0.717, P = 0.000). The concentration of NE in BALF was positively correlated with the duration of mechanical ventilation, CPIS and mortality (rs = 0.731, 0.528 and 0.680, P = 0.000, 0.001 and 0.000), while was negatively correlated with oxygenation index (rs = -0.729, P = 0.000). Taking serum NE > 150 μg/L as a positive index, the sensitivity was 83.72% and specificity was 94.37% in diagnosing VAP; taking BALF NE > 170 μg/L as a positive index, the sensitivity was 87.21% and specificity was 96.48% in diagnosing VAP. Conclusions NE has high sensitivity and specificity in the early diagnosis of VAP; the higher NE concentration in serum and BALF of patients with mechanical ventilation, the more severe VAP and the worse prognosis of patients with VAP are.