In this paper, we propose a transmitter with 150ºdivergence angle and more than 90% uniformity of radiation intensity for quasi-omni-directional underwater wireless optical communication (UWOC) by applying freeform lens into LED array. We demonstrate its performance under different transmitting directions with 19Mbps data rate and 8m communication distance in an experimental tank whose volume is 20m×20m×14m and attenuation coefficient is about 0.40m−1. The result shows that the variation of bit error rate (BER) is about three orders of magnitude with the help of freeform lens when direction deviation angle is between -75ºand +75º, which is reduced five orders compared with the situation without the lens. The presented result is useful to make it easier to establish communication link and improve the performance of UWOC system when transmitter direction shake exists. Also, it implies that we could apply different freeform lenses into LED array to reshape transmitting illumination according to different demands for UWOC application. © 2018 Elsevier B.V.