Objective To study the effect of soybean trypsin inhibitor (STI) on the rat islets’ yield and function during the process of isolation and purification. Methods The rats were divided into experiment group and control group according to whether STI was put into the collagenase. STI (2.0 mg/ml) was put into the collagenase digestive juice of the experiment group and none to the control group. For both of two groups, islets were isolated by situ perfusion collagenase into the rat pancreas and they were purified by un-continuous gradient centrifugation with Ficoll-400. The quantities of the obtained rat islets before and after purification were recorded, and the morphosis and function of the purified rat islets were tested, then their vivo function were observed after islets plantation. Results After digest and before purification, there was no obvious deviation of the obtained islets quantity between two groups 〔(624±38.2) IEQ vs (586±37.7) IEQ, P＞0.05〕; After purification, there were significant deviation in the islets quantity 〔(408±28.3) IEQ vs (189±27.1) IEQ, P＜0.05〕 and purity quotient 〔（93±2.4）% vs （75±2.1）%, P＜0.05〕. For two groups, there was no obvious deviation of the obtained islets in insulin stimulation and secretion experiment as well as their vivo function experiment. Conclusion The ultimate yield and purity quotient of the rat islets can be obviously improved by using collagenase digestive juice with SIT in situ perfusion on the rat pancreas, and it has no obvious effect on the islets function.