The internal leakage of the oscillating vane motor directly affects its output angular velocity; the transfer function of the oscillating motor with bypass leakage channel is established. By analysing the influence of the parameters of the bypass leakage channel on the minimum steady motor speed, the relationship between the minimum steady motor speed, the internal leakage of the motor and the parameters of the bypass leakage channel is given, and the calculation method of the input flow rate for the minimum steady motor speed is proposed.