Abstract: At present, the correctness of the web service composition is primarily analyzed by soundness and determining whether the composition model meets the behavior relativity is an effective method for judging soundness.But the algorithm for judging the behavior relativity is feasible only in the case that T-invariant exists.In the consideration of the fact, web services are modeled by the open Petri net.Then, in order to realize the correctness analysis of the composition, an algorithm for determining the weak soundness of the service composition based on the theory of the service tree is presented.Finally the concrete example shows the effectiveness of the proposed method.