[Objective] To solve the problems of reduced control accuracy in permanent magnet synchronous motor (PMSM) speed control systems due to variations in motor parameters and external disturbances, a model-free sliding mode control method for the PMSM speed loop based on adaptive gain is investigated. [Methods] The mathematical model of PMSM under internal parameter changes and load disturbance was constructed. The adaptive gain power reaching law was selected and combined with the model-free control concept to design a model-free sliding mode controller containing a disturbance term. The total motor disturbance was observed using a novel adaptive gain super-twisting sliding mode observer. [Results] With the help of Matlab/Simulink software and dSPACE platform for simulation and experimental comparison, the results demonstrated that the proposed method reduced the dynamic response time of the PMSM system, decreased the rotational speed fluctuation and the system chattering, and improved disturbance rejection ability and stability. [Conclusion] The proposed method can effectively improve the control accuracy of the PMSM speed loop, enhance the dynamic response of the system, inhibit chattering, and improve robustness.