The blue-green spectrum in the range of 450～550 nm is the only low loss optical window of seawater. Due to the absorption and scattering of water, the underwater propagation of the laser beam will have time-space expansion and waveform distortion. Based on Monte Carlo simulation, the propagation characteristics of blue-green laser in seawater have been analyzed, especially the influence of laser emission parameters on the distribution of receiving light field. By adjusting the beam parameters of the transmitter and focusing the laser beam in advance, the influence of beam diffusion caused by underwater transmission can be eliminated to some extent. The analysis and simulation results show that when the pre-focusing angle is equal to the small angle scattering eigenvalue of water, a flat transmission area with slow spread of laser beam can be formed in Jerlov type IB water before the designed focus distance. When the pre-focusing angle is greater than twice the scattering angle of water, the convergence effect can be formed near the designed focus distance, and the focus effect is proportional to the pre-focusing angle. When the water quality becomes worse, the corresponding effective focus distance becomes shorter. The research results provide a new way to improve the transmission characteristics of blue-green laser signal in seawater.