Due to the initial divergence angle of laser and the scattering of light in seawater,the size of laser spot in undersea transmission is changed,and the reception range is affected.The photons of laser with a wavelength of 514 nm transmission process is simulated in seawater base on the undersea wireless optical communication channel model.The spot expansion model in seawater is established by statistical photon number and photon position in different transmission distance,then the effect of seawater quality on the speckle space characteristics of Gaussian beam is analyzed.The results show that when the position of the light spot size is defined as intensity ratio,there is a linear relationship between the 1/e laser spot radius and transmission distance when the Gaussian beam transmission in the seawater,and the linear increase coefficient is the divergence half angle of laser source;when the receiver sensitivity is determined,the light spot decreases after a short distance;the light spot size is determined by the receiver sensitivity.With the increase of the receiver sensitivity,the size of the spot received is linearly increased at the same distance.