The processing conditions of soy protein isolate with neutral protease were optimized by response surface，which were: enzyme dosage 6%, hydrolysis time 6 h, substrate concentration 3.7%, temperature 42 ℃ and pH 7.0. The degree of hydrolysis was 7.74%. Five samples with different degree of hydrolysis, 3.1%, 4.4%, 5.2%, 6.2% and 7.4%, were prepared. Their variations of molecular weights were analyzed by SDS-PAGE. 2% of soy protein isolate or the hydrolysate samples were added into wheat flour, and the influences of the additions on dough rheological properties were investigated by Brabender farinograph and extensograph. Results showed that two new peptide fragments emerged after enzymatic hydrolysis of soy protein isolate with molecular weight about 30.5 ku and 26.4 ku; the water absorption of dough, stability, tensile energy, resistance to extension and stretching ratio were all increased when adding soybean protein isolate; when adding the hydrolysates, they were all decreased, and the dough rheology property got worse and worse with the increase of hydrolysis degree.