Ultrathin c-Si solar cells with a new light trapping scheme, which is a double-microcavity light trapping structure, are optimized and analyzed. The results show that by optimizing the geometric parameters of each light trapping unit, the light trapping structure can effectively improve the light absorption efficiencies of ultrathin crystalline silicon cells with different thicknesses. The thinner the active layer of a crystalline silicon solar cell, the more obvious the effect of the light trapping structure to enhance the light absorption. When the active layer thickness of a crystalline silicon cell is 3 μm, the battery efficiency can reach 16.3193%.