Abstract: In the development process of automatic welding equipment which is used for the complex inner surface welding in petroleum drill special-purpose valve,the mathematic model of valve inner surface intersection curve was modeled,and the motion trajectory of welding gun was planned.The 3D model of the welding equipment based on Pro/Engineer working environment was established,the definition of the motion jointing and the parameters of servo motors were set,the virtual prototype motion simulation experiment of the welding equipment was run,the motion trajectory curve was created,and the simulation errors was analyzed.The correctness of modeling the mathematic model was validated,which provided the simulation technology supporting of virtual prototype for developing and manufacturing the welding equipment.