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This study aimed to investigate the bioactive polypeptides of dairy cows' placenta and establish an effective enzymatic hydrolysis evaluation index for preparing bioactive hydrolysates. Hydrolysed placental homogenates by trypsin and papain and evaluated hydrolysis time, substrate concentrations and Enzyme/Substrate ratio to hydrolysates extraction rate and also their reducibility by Box-Behnken central response model. Optimal hydrolysis conditions of trypsin and papain for the highest extraction rate and reducibility were as follows: hydrolysis time 5.78, 5.13 h and 6.18, 3.95h; substrate concentrations 30.21, 35.59 %, and 34.78, 22.04 %, E/S 3.24, 4.19% and 5.00, 3.64%, respectively. The corresponding extraction rates and Vit.C equivalent were 5.55% and 39.32 μg/mg for trypsin, 6.97% and 70.54 μg/mg for papain. Correlations between the extraction rate and reducibility were significant in trypsin but not in papain. Optimized hydrolysis conditions with the highest integrated extraction rate (I) of trypsin and papain were as follows; hydrolysis time 5.8 and 4.7h; substrate concentrations 34.98 and 34.03 %; E/S 3.33 and 3.66%, respectively and corresponding integrated extraction rates were 2037.92 and 4414.09 μg·g−1, respectively. These results indicated that papain has higher hydrolysis efficiency than trypsin and I value is applicable for evaluating bioactive enzymatic hydrolysis of the placenta of dairy cows.