With the thorough study of the ocean, increasing number of underwater working devices, people are increasingly concerned about underwater energy supply. In this paper, a flexible concentrator for underwater concentrating photovoltaic power generation was designed. First, we performed theoretical analysis and actual measurement of the deformation of the concentrator. Then, optical performance of the concentrator was simulated and experimentally tested. The variation of the performance parameters, concentration ratio, focal diameter ratio (ratio of focal length to radius), light receiving rate, and spot energy distribution with the concentrator's deformation rate were obtained. It was found that the experimental test results are consistent with the theoretical simulation results, and the concentrator has good performance in concentrating light.