Abstract: In this article, effects of positions of laser spots on the wall of a cavity on variations of non –uniform degree of illumination for capsule with time have been studied by both numerical simulation and qualitative analysis. The results show that the geometrical factors relative to the pole and the equator of the capsule vary with positions of laser spots in different patterns. According to the comparison between their quantity, the tends of the variation of non –uniform degree of illumination for the capsule with time can be divided in three types, which correspond to laser spots to be in three different regions, respectively. The results given by qualitative analysis are in good agreement with those given by numerical is simulation.