In the absence of disturbance information, the chattering effect in terminal sliding mode control of a permanent magnet synchronous motor (PMSM) becomes more pronounced. To reduce the impact of unknown uncertainties and disturbances on control performance, this paper presents an improved adaptive fast terminal sliding mode control method based on a disturbance observer. First, a mathematical model of the PMSM is developed, accounting for parameter uncertainties and load disturbances, and a nonsingular fast terminal sliding surface is designed to enhance the system’s response speed. A disturbance observer is then utilized to estimate system uncertainties and unknown disturbances, with an adaptive gain introduced in the sliding mode controller to compensate for estimation errors, achieving adaptive robust control without requiring a known upper bound for disturbances. This improved adaptive control strategy dynamically compensates for disturbances, strengthening the system’s adaptability to unknown disturbances. Simulation and experimental results demonstrate that, in the absence of disturbance information, the proposed method effectively suppresses chattering caused by sliding mode control, improves the robustness of the PMSM system, and significantly enhances control accuracy and dynamic performance.