Abstract: In this paper,firstly,based on the basic features of spacecraft orbit,the formulae are deducted which are those of the optimum angle between the solar direction and the normal direction of solar array of spacecraft and the sunshine factor of circular orbit;Then,according to theoretical analysis,the coefficients of efficiency of the solar array are derived;Finally,having the aid of computer algebra system MATHEMATICA,a direct,clear and simple method is set,which can be used for analyzing the optimum sunshine of the solar array.The results may be of reference value to selection of launching window of a spacecraft.