A fast phase retrieval method for annular pupil using spot image is proposed. The analytic formula to compute point spread function for annular pupil is derived. With this formula, the point spread function is represented as linear composition of polynomials, the coefficients of which are just the annular Zernike expansion coefficients of the pupil phase. The objective function and its gradient in phase retrieval for annular pupil are computed with this formula. It can avoid the Fourier transform and finite differences, which are necessary in common phase retrieval method, and decrease the computational burden in phase retrieval. The effectiveness of the proposed fast phase retrieval for annular pupil is demonstrated using numerical simulation and phase retrieval experiment. Experimental results indicate that the proposed method can improve the speed of phase retrieval for annular pupil effectively.