In the AC servo system, the output voltage is restricted by inverter output limitations. Voltage saturation occurs as a result of these limitations. The settling time of the current and speed increases because of voltage saturation. This paper proposes a new control system for improving the transient response of the surface permanent magnet synchronous motor (SPMSM) by using the proposed flux-weakening control based on the current differential signal. Because the current differential signal is taken into consideration, the suppression of voltage saturation is achieved with a smaller d-axis current in a transient state. Finally, this paper proposes a new combination method of flux-weakening control and an inverter modulation scheme, which makes it possible to improve the voltage utilization on condition of flux-weakening region. The proposed speed control system achieves a quick and stable speed and current response. The effectiveness of the proposed control methods is confirmed by both the numerical simulation results and the experimental results.