In this paper, an optical transmitting antenna is designed using free-form surface design theory, which can generate a uniform rectangular spot to solve the problems of difficult alignment and poor spot uniformity in underwater wireless optical communication systems. The simulation results show that when the distance of the target plane is 10 m, the free-form lens can produce a rectangular spot with 87.3% uniformity and the light energy utilization rate is 93.4%. The free-form lens is applied to the underwater transmitting antenna, when the receiver sensitivity is -?30 dBm and the receiving aperture is 50 mm, the maximum communication distance of the turbid water is 2.3 m, and the maximum communication distance of the pure and clear sea water is greater than 10 m. Compared with the single antenna, the edge uniformity of the spot produced by the 360° omni-directional optical transmitting antenna composed of three sets of the free-form lens increases from 17.4% to 35.2%. The transmitting antenna improves the stability of the underwater optical communication link and provides a new technical idea for solving the alignment problem in underwater wireless optical communication.