We conduct a systematic study of thin film solar cells consisting of a GaAs needle profile (NP) grating structure as a light-trapping layer. The influence of geometric parameters on the optical absorption of the NP grating is investigated using rigorous coupled wave analysis and the finite element method. This type of structure can lead to broadband optical absorption enhancement throughout the wavelength range that we studied. Our simulation results reveal that the absorption efficiency of NP grating can be improved significantly compared with its rectangular grating counterpart. The proposed structure is expected to illuminate the design and fabrication of high-efficiency solar cells.