With the increasing demand of the resolution, synthetic aperture optical system is more and more important in the field of space observations, the cophase of its sub exit has become the main factor that restricts its application, the traditional phase diversity method can detect wavefront error, but the complex structure of objective function, low precision and high computational complexity do not meet the practical needs. In this paper, we construct the phase diversity model by using the properties of Zernike polynomials, expand the phase by first order Taylor series and solve least squares problems to replace the nonlinear optimization problem. When the wavefront error is small this method can accurately recover the wavefront phase. Experiments show that the method can correct the phase error better than 1/60λ and achieve satisfactory image restoration results.