Abstract: The multiple scattering has happened when the laser pulse of the underwater lidar by wake is in the near-field transmission. Its coaxial optical system has a certain near-field suppression ability to the strong scattering and can prevent saturation of the receiver system. The near-field suppression ratio of the coaxial optical system is of great significance. The reason for that the traditional formula of photon returned contribution was not suitable for the near-field calculation was analyzed and a new formula was presented for underwater near-field coaxial optical system. A simple method to solve it with coordinate transforms was presented. The near-field suppression ratio was calculated based on Monte Carlo simulation. The relationship between near-field suppression ratio and suppression time and the relationship between near-field suppression ratio and the laser pulse width were analyzed and calculated. The results prove that this method can satisfy the needs of underwater coaxial optical system analysis completely.