Under asymmetric information environment, the evolutionary game model for opportunism behavior in purchasing alliance is established respectively in circumstances without preventative measures and introducing trust mechanism. Then, relevant conclusions are proved, and verified by means of numerical simulation. The result shows that, (1)The opportunism behavior will flood in the alliance without any preventive measures, and ultimately purchasing alliance will disintegrate. (2)Introducing trust mechanism, the opportunism behavior can be effectively restrained, when information cost is less, the opportunism behavior proportion is lower, trust and penalties are moderated; with few information cost, the opportunism behavior also can be effectively restrained through lower trust, higher penalty and more moderate adjusting purchasing enterprise and alliance factors, such as enterprise strength, learning ability, cooperation degree, cooperation intention, transparency and cooperation risk.