Aiming at the problem of poor performance of traditional predictive function control (PFC) in speed control due to external disturbances, parameter mismatches, and measurement and control quantization errors caused by position sensors in the servo control system of permanent magnet synchronous motor (PMSM), an optimized predictive function control method was used by using a composite adaptive position observer. The composite adaptive observer was constructed by combining model reference adaptive control (MRAC)  with extended state observer (ESO) to improve the locationless observer observation accuracy and disturbance resistance, accurately detected rotor information, and used it as feedforward compensation for the predictive function controller, which achieved fast and high performance control of the overall servo system. The experimental results show that the improved  predictive function control has feasibility and superiority in response speed and disturbance suppression under sudden changes in load and speed.