This paper presents a simulation system based on STEP-NC that can verify STEP-NC program and demonstrate the running of STEP-NC system (CNC system based on STEP-NC). NC Programs used in the STEP-NC system are in XML format. The overall architecture of the system which consists of feature extraction module, graphical modeling module, movement modeling module, feature library, simulation drive module and user interface is established. Some key technologies to implement simulation system are discussed in details. The simulation system shows great potential to the development and popularization of STEP-NC system.