In this paper, polymerization of acrylic acid was induced by excimer laser (248 nm). The molecular weights of polymers of acrylic acid were measured by GPC gelatin chromatogram. The rule of polymerization of acrylic acid was also explored. The results of experiments show that the polymerization is a kind of radical induced polymerization adding H 2O 2 as photo initator. In the experiments, the molecular weights of polyacrylic acid can be increased when the concentration of acrylic acid solution is diluted; the laser irradiating time decrease and the power of each pulse increases.