Since accurately identifying the error source which causes the exception is important, the problem of diagnosis for composite service becomes one of the key issues in the adaptive service composition. This paper proposes an approach for diagnosing composite service. In this approach, the uncertain casual relation between the exception and the service is formed by the way of computing the error propagation degree. The diagnosing algorithm which is based on the error dependent matrix is established. The diagnosis can be achieved by fuzzy reasoning. This approach can preserve the efficiency and accuracy of the diagnosis for composite service, which can support the self-adaptation of the composite service effectively.