Abstract: Investigations on the extinction properties of light in seawater has farreaching significance in the field of underwater detection. Proceeding from the analysis of absorption and scattering properties of light in seawater, the extinction properties of inorganic salt solution, suspend particles and yellow material are emphatically discussed. It is shown that the main factors of the absorption of light in seawater are pure water, plankton and yellow material. The inorganic compounds mainly account for the scattering effect. The spectrum extinction properties of different seawaters are measured by spectrophotometer. In comparison with the pure water, among Hulu Island, Hainan Island and Lianyun Port, the extinction index of the seawater in Lianyun Port is biggest, and Hulu Island is weakest. Thus, inorganic salt solution, suspend particles and yellow material can be taken as the main components that affect the extinction properties of light in seawater.