Abstract: Taking the ashtray for example，the whole process of NC machining simulation of cavity part was described.Firstly，Master CAM was used to complete the modeling of cavity part，rough setting， tool path setting and NC program.Then，the program was edited.Finally，simulation of NC machining process was completed with the Guangzhou NC simulation software.The simulation results showed that this method could complete the entire process from the geometric modeling to simulation processing，the final results was almost ideal.