Under the condition of adding chitosan， investigate the protective effect of skimmed milk powder， sugar， maltodextrin， maltose， lactose， trehalose， sorbitol， ascorbic acid 8 kinds of freeze-drying protective agent on the freeze-drying process of black soybean nattokinase. Through the determination of the rate of nattokinase enzyme activity and survival Bacillus natto， finally determination of four combinations of compound freeze-drying protective agent. The addition amount is as follows： Skim milk powder 16%， ascorbic acid was 1.2%， the malt was 8%， sucrose 8%. Under this condition， the rate of nattokinase enzyme activity is 79.8%， and the rate of survival Bacillus natto is 83.2%， the effect have being improved a lot than a single protectant.