Objective To investigate the effects of dilute concentration and acting time of pronase on quality of gastroscopy.Methods A total of 448 patients were randomly divided into two groups : sodium bicarbonate group and pronase with sodium bicarbonate group.Pronase was diluted into 50 ml (400 U/ml)and 100 ml (200 U/ml) using sodium bicarbonate.The patients were pretreated by pronase of different concentrations 10 min, 20 min, 30 min, 60 min and 120 min before gastroscopy.Diluent of same quantity were taken by the control group.Visibility of gastroscopy, procedure times and positive rates of lesions were compared.Results Pretreatment of pronase significantly improved visibility of gastroscopy, raised positive rates of lesions, and reduced procedure times of gastroscopy, compared with the control group (each P ＜ 0.05).The visibility of gastroscopy were over 80％ 20,30, and 60 minutes before the examination with no significant difference(P ＞ 0.05).The visibility of gastroscopy decreased sharply 30 minutes after taking pronase, especially after 60 minutes.There was no significant difference in the quality of gastroscopy between the 200 U/ml and 400 U/ml group 20-60 minutes before gastroscopy (P =0.640).Conclusion Pronase (200 U/ml-400 U/ml) significantly improves visibility of gastroscopy, raises positive detection rates of lesions, and reduces procedure time of gastroscopy 20-60 minutes before pretreatment.