Using the fundamental equations of solar radiation received by a horizontal plane and an isolated inclined plane，we have discussed the characteristics of solar radiation incident upon the polymer specimen mounted on the exposure rack．If solar radiation is the main factor influencing the weathering of polymers，we suggest that the exposure rack should be set according to the following angles facing south in different seasons：where aM1.2，αM3.4 and αMg are the optlmum exposure angles to give the maximum rate of outdoor weathering in different seasons，and φ is the geographical latitude of the location．