Objective To study the effect of water soluble chitosan (WSC) on the apoptosis of peritoneal macrophage induced by lipopolysaccharides (LPS), and discuss the mechanism. Methods Peritoneal macrophages were divided to three groups: phosphate buffered saline (PBS) group, LPS group and LPS plus WSC group. At hour 24, apoptosis cell and active caspase-3 were detected by flow cytometry; nitric oxide (NO) was determined with Griess reagent. Results There were more apoptosis cells in the LPS group than the PBS group. The percentage of apoptosis cells was significantly decreased in the LPS plus WSC group than the LPS group. The expression of active caspase-3 and the secretion of NO were also inhibited by WSC after LPS intervention. Conclusion WSC inhibits apoptosis of peritoneal macrophage induced by LPS.