There is a problem of poor control accuracy of speed-sensorless control of an asynchronous motor due to inaccurate estimation of rotor flux. Thus an improved hybrid model rotor flux observer design method is proposed. First, the conventional flux observation method of an asynchronous motor is introduced, and the main problems of the conventional Gopinath rotor flux observer are analyzed in detail using the complex vector model and characteristic function of asynchronous motor. Secondly, an improved characteristic function design method is proposed to solve the drawbacks of the conventional rotor flux observer, and then an improved hybrid model flux observer is obtained. Thirdly, the parameter sensitivity of the conventional Gopinath rotor flux observer and the improved hybrid model rotor flux observer are further compared based on the characteristic and frequency response functions. Finally, the speed-sensorless control of the asynchronous motor is realized using the flux observation results of the proposed hybrid model rotor flux observer and the phase-locked loop. The comparative experimental results based on StarSim and YXSPACE-SP2000 experimental platforms verify the parameter robustness of the hybrid model rotor flux observer proposed, as well as the effectiveness of the proposed speed-sensorless control strategy.