The zenith and azimuth angles of the surface cells of the earth are sampled uniformly. The position of a surface cell is transformed from the sphere reference frame into the earth equatorial reference frame. The geometric factors of earth radiation and earth's reflective radiation are calculated through numerical integration. The calculation result shows that this method is better than the random simulation method in calculation precision and efficiency and its data is in most agreement with that in the radiant exchange heat manual. It is very suitable for the calculation of the heat flux of a target.