This paper presents a deformable underwater solar concentrator (DUSC) which is innovatively designed to converge underwater radiation. The concentrator is a hollow cylinder-like structure with elastic films covering the two flat ends. When it is put underwater, the elastic films will concave inwards and form spherical convex lenses. The design scheme and concentrating peculiarities of the concentrator were analysed. Optical simulation and experimental verification were carried out on a concentrator sample with diameter of 200 mm. It is found thatf-number (the ratio of focal length to radius) and concentration ratio will decrease when enlarge the films' deformation ratio. The effective deformation ratio is found to be less than 0.45 to avoid total reflection. Optical simulation under condition of visible spectrum with considering the solar parallax indicates that the maximum energy density of the focus occurs when deformation ratio is 0.3, andf-number is around 3.5. During the tilt incidence, the shape of focus and energy density value will not vary distinctly until the tilt incident angle is above 10 degrees. The average experimental optical efficiency of the DUSC is about 60% when incident angle is within 10 degrees. This work is likely to provide a new perspective for utilizing solar energy underwater.