This paper investigates the problem of rotor flux observation in asynchronous motors based on the super-twisting sliding mode algorithm. According to the asynchronous motor current model, we design the open-loop estimation model of rotor flux under the super-twisting algorithm. In order to improve the robustness of the model to rotor resistance and external perturbations, the uncertainty part of the motor parameters in the observer is estimated. Furthermore, the rotor flux obtained by the observer is introduced into the stator current estimation equation, a closed-loop structure is constructed to improve the accuracy and convergence speed of rotor flux observations. By reconstructing the intermediate variables to be observed in the model, the problems of integral drift and initial value error are figured out on the traditional observer model for solving the rotor flux. For the coupling problem of flux and rotor speed, a sufficient condition for the stability of the system is given by dealing with the perturbation. Simulation and experimental results verify the feasibility and effectiveness of the algorithm in this paper.