In order to study the mechanism of protease which influences on the flavor of soymilk, different kinds and concentrations of protease (papain, 15000, 30000, 45000, 60000 and 75000 U/L, aminopeptidase, 500, 1000, 1500, 2000 and 2500 U/L, flavor protease, 100, 150, 200, 250 and 300 U/L, alkaline protease, 500, 1500, 2500, 3500 and 4500 U/L) were added to the soymilk made by automatic soymilk maker. Sensory evaluation was carried out on the soymilk after heating. The content and difference of polypeptide and amino acid nitrogen in the soymilk after enzymatic hydrolysis and heating were determined, and the correlation between them and sensory flavor indexes was analyzed. The volatile flavor components of soymilk were analyzed by headspace solid-phase microextraction combined with gas chromatography-mass spectrometry (GC-MS). The results showed that in the blank control group prepared by automatic soymilk maker without protease, the unpleasant beany odor and pleasant bean aroma of soymilk after heating were both lighter than those prepared by traditional method named the raw bean control group, while the contents of polypeptide and amino acid nitrogen showed reductions of 37.98% and 25.78% as compared to the raw bean control group, respectively. The sensory evaluation was carried out after adding different proteases to the soymilk prepared by automatic soymilk maker. The results showed that the soymilk added with 250 U/L flavor protease had the highest sensory score, which increased by 118.41% compared with the blank control group. In all groups added with proteases, the sensory score was significantly higher (P<0.05) except when it was added with 4500 U/L alkaline protease, compared with blank control group. The contents of hexanal, 2-pentyl furan, hexanol, trans,trans-2,4-decadienal and other volatile compounds related to beany ador in the enzyme added samples were lower than those in the blank control group and the raw bean control group, and the contents and types of volatile compounds related to bean aroma were increased to varying degrees. The addition of exogenous protease could significantly improve the contents of polypeptide and amino acid nitrogen in soymilk, among which the content of polypeptide was the highest when adding 4500 U/L alkaline protease (2.30±0.15 g/L), and the content of amino acid nitrogen was the highest when adding 2500 U/L aminopeptidase (0.087±0.0045 g/L). The amino acid nitrogen content of enzymolized soymilk in the group added with flavor protease was significantly positively correlated with the score of bean aroma (P<0.05), the increase of polypeptide in groups added with the other three enzymes were significantly or extremely significantly positively correlated with the score of beany odor (P<0.05, P<0.01). The increase of amino acid nitrogen was significantly negatively correlated with the beany odor, bean aroma and total score in the group added with aminopeptidase (P<0.05). The above results indicated that proteases are beneficial for increasing the content of polypeptides and amino acids as flavor precursors, which give a lighter beany odor and a richer bean aroma to the soymilk. This study provides theoretical guidance for the optimization of soymilk preparation technology.