A small-aberration retrieval method based on spot images was introduced and an analytic formula to computer the point spread function for a small aberration optical system was derived according to the scalar diffraction theory and Bessel series. With this formula, the objective function and its gradient were given for the small-aberration retrieval method. Then, the optimized method was used to solve this problem. The effectiveness of this method was demonstrated by using the numerical simulation under ideal and noisy conditions，respectively, and the influence of the size of spot object on the retrieval results was also analyzed. This method uses aberration's Zernike coefficients as optimization variants and computes the objective function and its gradient with analytic expressions to avoid Fourier transform or numerical integral needed in common aberration retrieval methods. Therefore, it improves the speed of aberration retrieval greatly.