Torque fluctuation coefficient is an important index to measure the quality of brushless torque motor. In the process of development and production of motors, the measured value of the torque fluctuation coefficient of the prototype is higher than the designed value. Through the prototype test and simulation, the causes of the difference are analyzed from the aspects of winding coil, three-phase currents, and eccentricity of stator and rotor in the production process of the prototype. The results show that eccentricity is the main reason for the measured value to be higher than the designed value, while three-phase current and winding coil have less effects on the difference. By optimizing the structure design of the motor to reduce the eccentricity of stator and rotor and correcting the design value according to the torque fluctuation caused by the eccentricity, the consistency between the design value and the measured value can be improved.