The refractive error of ground-based laser attacking space targets such as satellites is discussed. The refractive index of atmosphere is variable according to the height. Based on the Snell law, the refractive error due to laser beam going through atmosphere is studied. The relationship between the refractive error and the initial elevation of laser beam is calculated by an example. According to the result, the refractive error is big when the elevation is low and the refractive error is small when the elevation is high. At last, some problems are also raised.