The channel characteristics were investigated for underwater laser communications based on Monte Carlo simulation method. Using the blue-green laser with wavelength 532 nm, the impulse response of link was discussed both in theorem and simulation. Also the channel parameters such as the water type, attenuation length, divergence angle, beam width, received field-of-view(FOV), received aperture were taken into account. Monte Carlo simulation experiment was verified. Simulation results suggest that in clear water, the channel can effectively be considered as non ISI when working distance less than 40 m. Therefore, it practically did not need to perform computationally complex signal processing at the received end. However, in harbor water, the channel delay spread enlarged with larger received FOV or divergence angle, and the data transmission efficiency was reduced. When the attenuation length was smaller than the diffused length, the received intensity decreased with received aperture enlarged. In contrast, the received intensity enhances with larger received aperture at the small FOV, however, they trend to similar regardless of the apertures at large FOV. Therefore, the results give us some insight in terms of what constitutes an accurate underwater laser communication channel.