The speed-regulation problem for a permanent magnet synchronous motor (PMSM) servo system is studied in this paper. In traditional control design for a speed loop, a first-order model is used to approximately describe the relationship between the reference quadrature axis current and the speed output, i.e. the reference quadrature axis current is regarded as the same as the quadrature axis current. This approximation degrades the closed-loop performance of PMSM system when using a vector control scheme. To this end, a second-order model is built to describe the relationship between the reference quadrature axis current and the speed output for a PMSM system. Based on this second-order model, a standard sliding mode controller is designed for the speed loop. Then, to reduce the chattering and improve the performance of the system, a composite controller consisting of a sliding mode feedback part and a disturbance compensation part based on an extended state observer are developed. Simulation and experiment results and comparisons are given to show the effectiveness of the proposed method.