Soybean protein isolate hydrolysate ( SPIH) obtained at different enzymatic hydrolysis times ( 3，12 and 24 h) were added to homemade yoghurt． The effects of SPIH on the fermentation and storage quality of yoghurt were studied by measuring the enzymatic hydrolysis characteristics，fermentation characteristics and storage quality of SPIH ( SPIH-A3，SPIH-A12 and SPIH-A24) ． The results show that the longer the enzymatic hydrolysis time，the higher the degree of SPIH hydrolysis． All SPIH can promote yoghurt fermentation，inhibit post-acidification during yoghurt storage，reduce whey precipitation，and reduce the number of viable bacteria． Ultrafiltration separation was carried out on SPIH-A12，which has the most significant effect of promoting fermentation． It was found that the SPIH-A24 component below 4 ku had more hydrophobic amino acids and uncharged hydrophilic amino acids，which could improve the number of live bacteria and the apparent viscosity of yoghourt coccus and reduce the whey precipitation rate． Appropriate enzymatic hydrolysis of SPIH can effectively promote yoghurt fermentation，improve yoghurt storage quality． In the meantime，small molecular fraction obtained by ultrafiltration demonstrated a more pronounced effect．