The fishnet metastructure has plane, near-optical lossless characteristic, and can excite surface plasmons in a specific light field. It has great potential in enhancing the response efficiency of photonic devices. Based on the finite difference time domain method and rigorous coupled wave analysis, in this paper, we systematically study the plasmon resonance mode of the fishnet metastructure and its light wave regulation performance on the crystalline silicon thin film solar cells. The research results show that the characteristics of absorption, scattering and extinction for the fishnet structure strongly depend on the thickness, line width, period and other characteristic parameters of the metal layer. Through optimizing the design, the resonant peak is red-shifted to 770 nm, and the relative extinction cross-section reaches 1.69, and the scattered light occupies a dominant position in the extinction spectrum. The crystalline silicon thin film solar cell with a response layer thickness of 2 μm constructed in this way has a significantly enhanced absorption efficiency in the wavelength band greater than 800 nm, and the final energy conversion efficiency of the device increases from 6.67% to 8.25%. The light intensity distribution shows that the enhanced backscattering caused by resonance and the large-angle deflection of the photon propagation direction are important reasons for the response gain of the solar cell.