Ultra-thin crystalline silicon (c-Si) solar cells have the advantages of flexibility and light transmittance. The surface etched texture structure for reducing reflectivity usually sacrifices the flexibility of the solar cells. In this work, a kind of polydimethylsiloxane (PDMS) film with Moth-eye nanostructure was introduced to the surface of flexible ultra-thin c-Si solar cells, which was proved to enhance the flexibility of the solar cells besides reducing reflectivity and improving conversion efficiency. The finite element method was used to study the photoelectric and mechanical properties of the solar cells, the results show that the PDMS film has good flexibility to protect the ultra-thin c-Si surface, resulting in a 33.4% reduction in surface stress. At the same time, the refractive index of PDMS film with Moth-eye nanostructure has gradient change in the direction of incident light and is between that of air and c-Si. The PDMS film with Moth-eye nanostructure, as the antireflective layer on c-Si surface, reduces the reflectivity by 46.4% and improves conversion efficiency of ultra-thin c-Si solar cells by 42.3%. This work provides a method for other flexible solar cells to improve flexibility and reduce reflectivity.