A novel continuous revolving electro-hydraulic servo cam-rotor motor was introduced with its operating principle and features. The mechanism of the disturbing torque of the existing motor was found based on systematical force analysis of the vanes. The rules of  positive pressure of  the motor’s cam and disturbing torque with changes of oil pressure from the reducing valve were obtained by computer simulation. As disturbing torque would cause the speed fluctuation of the motor, the error band of the error curve of motor’s ramp signal tracking was chosen as the indicator of the scale of motor’s speed fluctuation. The comparisons of error band at different oil pressure from the reducing valve demonstrated that the speed ripple of the motor was more intensified with the increase of the reduced pressure.