Soy protein and polyvinyl alcohol degradation mechanisms were described separately. With weight loss rate as the evaluation index of degraded performance, studied the degradation of soybean protein / PVA composite film and transglutaminase modified soybean protein / polyvinyl alcohol composite film under outdoor natural condition and indoor constant temperature and humidity condition. Experimental results show that: TG has little effect on the degradation of soybean protein / polyvinyl alcohol composite film. When the degradation time arrived the 9th day, the weight loss rates for two kinds of films are both above 60% either at indoor constant temperature and humidity condition or at outdoor natural condition. When the degradation time reached the 161st day, the weight loss rates of soybean protein / polyvinyl alcohol composite film and TG modified soybean protein / polyvinyl alcohol composite film at indoor constant temperature and humidity condition reached 64.0% and 64.1% respectively and at outdoor natural condition they were 66.0% and 67.0%. TG can improve the mechanical properties of soy protein / polyvinyl alcohol composite film and does not affect the degradation performance.