Underwater wireless optical transmission faces the problems of power attenuation and time domain width enlargement， the long-distance transmission characteristics of blue-green light are also more difficult to test. Monte Carlo method is used to investigate the process of underwater Gaussian beam， the effects of channel parameters such as water type， attenuation distance and divergence angle on the received power and impulse response are taken into account， power distribution on the receiving surface with different seawater types and transmission distances under small divergence is also numerically compared. The simulation results show that the received power decreases gradually and the time-domain width expands greatly with the increase of seawater attenuation coefficient， attenuation distance and divergence angle， among them， the influence of divergence angle on the transmission distance of turbid harbor water above 10 m is small， while the width of time domain changes from 0.32 ns to 0.8 ns with the increase of divergence angle. Although the transmission distance is doubled， the power similarity is still up to 99% in pure sea water and clear ocean water. Then based on the difference of seawater attenuation coefficient， we propose the power similarity and area ratio to analyze underwater long-distance transmission， so as to obtain the long-distance transmission characteristics more efficient and quicker.