With respect to stress condition of external circulation rotor pump during operation, through stress analysis of pump rotor and shaft, a mechanical model was built up and the calculation formula was derived for the gap between the pump rotor and pump body; according to Navier-Stokes differential equation, the leakage loss equation for the gap of rotor pump was deduced, and through comparative tests, the practicability of the calculation formulae for the gap and leakage loss were verified, which provided a theoretical guidance for improvement of design level and performance of the pump.