Recently research on modeling methods of complicated processes under complex network environments has become a focus in workflow field. Now cloud computing environment provides a specific application background for the interaction and collaboration of workflows. In this paper, workflow processes were modeled as Semantics Constrained Net (SCN), which is one kind of extended Petri net. Then net synthesis mechanism constrained by semantics was researched, including synthesis rules and liveness proven. And a scalable solution of complex workflow process modeling in cloud was proposed. An instance of e-business was given to describe cooperative workflow under cloud computing environment.