Abstract: Due to the fact that single Web service is too simple in function to accomplish complex business requirement, a coordinated aggregate method (service composition) is introduced to build enterprise solutions by combining multiple existing enterprise services. A composition of services is comparable to a traditional application in which its functional scope is usually associated with the automation of a parent business process.Based on XML, BPEL uses XPath to bind variables and define expressions. Different from WSDL which can only define simple constraint by pre-and post-condition, XPath expressions combined with structured activities provide the more expressive ability to define constraint.Both white-box testing and model checking are based on BPEL modeling and feasible path analyzing. For BPEL, only considering the structural activities and ignoring the data manipulation and constraints will bring some negative impacts.To address this issue, we propose an XPath expression oriented predicate constraint analyzing and modeling approach and introduce a feasible path analysis algorithm based on this model. This approach takes into account the data manipulation effects on the feasible path. We firstly analyze the syntax of BPEL expressions and make a normalizing to the BPEL expressions. Then, the activity effect extended variable structure tree is used to model the atomic expression and the composite predicate expression. Moreover, the feasible path analysis algorithm is discussed using the established model. Finally, by case studying, the feasibility and experiment process of our approach are illustrated.