A structure of one-dimensional bilayer cosine conformal grating crystalline silicon thin film solar cell was designed. The absorption spectra of bilayer cosine conformal grating structure and contrast groups were simulated by using the finite difference time domain method ; The absorption efficiencies of two wavelength regions of 300 ~ 700 nm and 700 ~ 1100 nm were quantitatively analyzed by using the concept of normalized optical absorption density. It was found that bilayer conformal grating structure has better absorption efficiency in full wavelength band and cosine grating has better light trapping and absorbing than rectangular grating in long wavelength region. To analyze the mechanism of optical absorption enhancement, absorption enhancement spectra and distributions of electromagnetic field intensity were plotted. Through the calculation of short-circuit current density, it was found that the short-circuit current density of bilayer cosine conformal grating crystalline silicon thin film solar cell increased by 79.5% relative to planar structure, the short-circuit current density of cosine grating structure increased by 8.5% relative to rectangular grating structure.