As enterprises continue to improve the degree of information technology,business information systems integration is irresistible,however risk inheres in integration when the integration is carried out.Aiming at information systems integration for elements and processes,the risk assessment of information systems integration project by Bayesian method was proposed and the integration expenditure for risk factors was analyzed.The analysis shows that Bayesian method is perfect in assessing information system integration risk.Especially if the expertise experiences and the premise value are available in priori,the risk assessment can get good effect.