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Business Process Execution Language has become one of the most important technologies in Service Oriented Architecture, and enables easy and flexible composition of services into business processes. In practice, many business process scenarios require human interactions. WS-BPEL Extension for People (BPEL4People) specifies a BPEL extension to address human interactions in BPEL as a first-class citizen. However the need for detecting the schedule problem is beyond what the BPEL4People can handle. Given a BPEL process with BPEL4People extension, we analyze and solve the schedule problems with a formulized approach. We first transform a BPEL description into a service workflow net, which is a kind of time Petri net (TPN). Based on this formalism, we propose algorithms to generate the Reachable Marking Graph (RMG), analyze the schedule problem, and then provide a method to create new markings to solve the timing conflicts in people activity. Our approach is validated through a real-life case and further research directions are pointed out.