In order to investigate the system dynamics of spacecraft propulsion system with complex pressurization pipelines and propellant feed pipelines, the Unified Propulsion System simulator (UPSSim) was developed, which consists of a modular and extensible framework based on AMESim software. The pressurant pipelines were separated into several nodes, with each node using lumped parameter model; while the propellant feed pipelines used distributed parameter model. Heat transfer between components and environment was also taken into account. Simulation of a certain UPS during GEO insertion shows that the UPSSim can obtain the parameter variations accurately, and the results are in good agreement with design parameters and measurements. Simulation result indicates that the method proposed is sufficient for the modeling of complex pipelines and has good application prospect for spacecraft propulsion system dynamic simulation. 