The tooth profile lines of the inside circulation piston pump were studied, and their meshing characteristics were analyzed. According to the motion law of the inside circulation piston pump rotor, and based on the tooth profile lines of the inside rotor as the circular arc, the moving coordinate system was established. Assumed that the outside rotor was fixed and the inside rotor made the relative movement, the calculation formula of the outside rotor profile was deduced for the first time. At last, using an actual design example and motion simulation analysis, the validity of the tooth profile line equation and calculation formula was verified. 