Analysis of phase difference of spherical laser window is presented, which includes unequality of optical path caused by window itself and disuniformity of refration index and strain caused by thermal and photoelastic effects. The distribution of phase difference is calculated with differential method. It indicates that choice of window parameters influence the distribution of phase serious. The optimal parameters of laser window can be given based on thermal and photoelastic effects according to center intensity of far field beam to minimize the influence on beam quality.