Using the Monte Carlo method, we investigate the transmission characteristics of light beam through seawater for long-reach undersea wireless optical communication system. The traditional Gaussian beam is replaced with flat-topped beam, which can extend the distance of communication system. Then we compare the transmission characteristics of flat-topped beam and Gaussian beam in three kinds of clear seawater, and simulate the two kinds of underwater wireless optical communication systems based on pulse position modulation. The results show that the upper limit of reliable video transmission distance of undersea wireless optical communication system is 268 m for using Gaussian beam and 340 m for using flat-topped beam under the same condition, respectively. The application of flat-topped beam is viable to extend the communication distance of underwater wireless optical communication system in clear seawater. The results provide a reference for the design of long-reach underwater wireless optical communication systems.