The light propagation in water becomes complicated due to the scattering of water. Limited by optical thickness and other factors,the single scattering approximation is insufficient. Based on the radiative transfer equation and small angle approximation,the multiple scattering solve is gradually recursive to describe the whole scattering process. The calculation relation between underwater irradiance and the attenuation coefficient is deduced and the laser attenuation characteristic in seawater is discussed. The results show that,with the increase of optical thickness,each order scattering is first enhanced and then decreased,the relative irradiance attenuates exponentially,and the attenuation trend is declined when the single scattering albedo or asymmetric factor rise. Verified through Monte Carlo simulation method,the multiple scattering approximation method has a better accuracy than the single scattering approximation on the condition of small scattering angle and can be used as the reference for analysis of the underwater laser propagation.