Abstract: Platform as a Service (PaaS) is hotspots of enterprise cloud platform product selection and cloud technology research. This paper discusses PaaS compositing mode on the microscopic and macroscopic perspectives and proposes a cross-level method to select PaaS compositing mode by the compositing of PaaS platform based on SOA. Innovatively, feasible service compositions can be find by Petri net structural analysis and PaaS environment of all kinds of architecture model are simulated by CloudSim. We use Petri net and CloudSim interactive simulation to measure the technology properties of service compositions for PaaS compositing mode and apply Analytic Network Process (ANP) to evaluate comprehensive performances of PaaS compositing models. Consequently, this method can provides the guidance for PaaS platform selection and construction cloud service platform.