In this paper, we propose to analyze the cogging force of an arc permanent-magnet synchronous motor (Arc PMSM), which will be applied on the next generation large telescope. Finite element analysis (FEA) is carried out to analyze the cogging force of Arc PMSM by using planar cross sections. By adopting optimization methods, such as adjusting the length of stator and adopting fractional slot, the end cogging force decreased from 6.2 N . m to 2 N . m, and the slot cogging force decreased from 6.0 N . m to 0.67 N . m accordingly. A geometric optimization in regards to the relative positions of stators along the rotor is also contrived to reduce the end cogging force. Both simulation and experimental results show that the total toque ripple can be as little as 2%, which can satisfy the tracking accuracy of the telescope.