In order to solve difficulties in precision machining polishing to inner surface of circular pipe, an automatic type of the device was designed. Mechanical structure of the whole polishing device was composed of parts as polishing head, circular section bending, transmission, clamp and link mechanism, and etc. The whole device and the circular section bending parts were connected into one cavity, which was injected with fluid for inner surface polishing of the parts. Operation of the device of circular section bending parts at both ends was driven by stepper motor controller, thereby precision machining polishing to inner surface of circular pipe was implemented. Fluent software was used for fluid simulation. The simulation results show that the flow speed of fluid is very high at exit, which makes a well play of rotational injection polishing, and machining polishing for inner surface of the circular crosssection bending parts is very well accompanied.