【Abstract】 Objective To evaluate the efficacy and safety of two different angles of head of bed elevation (25°~30° and 45°) in the prevention of ventilatorassociated pneumonia (VAP) in adult patients with mechanical ventilation and to evaluate whether 25°~30° semirecumbent position is superior to 45°head of bed evaluation. Methods Randomized controlled trials (RCTs) on 25°~30° and 45° were performed from January 1, 1980 to May 1, 2017 at home and abroad, and collected by electronic and manual retrieval. The relationship of the angle of bed head evaluation and VAP incidence and other clinical prognosis were observed. Results 6 RCTs were eligible for inclusion, and 6 randomized trials (636 patients) were used for metaanalysis using the random effects model. The results showed that there was no significant difference between 25° to 30° of head evaluation in reducing the incidence of VAP ,compared with 45° ［relative risk (RR)=130; 95% confidence interval (CI) 091, 186; P=099; I2=0%］. Three observational studies were performed to show that bedside elevation of 25° to 30° might have a lower risk of pressure ulcer, better clinical compliance. Conclusion In adult mechanical ventilation critically ill patients, compared with the bedside elevation of 45 degrees, the bedside elevation of 25°~30°, there is no significant difference in preventing the incidence of VAP and other clinical complications, but it can improve the clinical compliance and reduce the risk of pressure ulcer.