For flux weakening control of the internal permanent magnet synchronous motors, when the degree of flux weakening is deeper, the motor parameter perturbation and external disturbances will cause the voltage loop output, torque and current pulsation to increase, and the speed convergence is too slow. A speed-voltage loop feedback super-twisting non-singular fast terminal sliding mode controller (FST-NFTSMC) is proposed for deep flux weakening control. To reduce the dependence of flux weakening control on the system model, the voltage-loop hyperlocal model is constructed according to the mathematical model of the built-in permanent magnet synchronous motor during parameter perturbation. And it is combined with the speed loop hyperlocal model to establish the speed-voltage loop hyperlocal model. Based on this hyperlocal model, the speed-voltage loop FST-NFTSMC is designed by combining the feedback super-twisting algorithm and the non-singular fast terminal switching function. At the same time, an improved sliding mode disturbance observer is built to estimate the unknown part of the system and compensate for the estimated value feedforward to FST-NFTSMC, which further improves the robustness and control accuracy of the system. Simulation and experiment show that compared with the traditional PI control, the convergence speed of the proposed method in the no flux weakening region, shallow flux weakening region, and deep flux weakening region is improved by 66%, 40.6%, and 28.6% respectively. It has better stability and fewer pulsations of the torque and current, proving that the method in the flux weakening control is effective in suppressing the output jitter after the voltage loop is perturbed as well as improving the speed response.