Cogging torque is one of the emphases and difficulties of research on permanent magnet (PM) machines. In this paper, an improved analytical model, based on an exact subdomain model idea and Maxwell stress tensor, is applied for calculating a cogging torque of surface-mounted PM machines with auxiliary slots. It can accurately take account of the interactions between slots and the influence of finite slot depth and can be widely applied to internal/external rotor topology and radial/parallel/segmented-Halbach magnetized machines. The finite-element (FE) method is adopted to validate the proposed model. The results show good agreement in both flux density and cogging torque waveforms. Furthermore, the effects of auxiliary slot parameters on cogging torque are investigated. Conclusions are derived from the analysis and indicate that the analytical model provides a useful tool for PM motor design.