In this paper, a metamaterial terahertz broadband reflector, which is composed of a middle layer of polyimide film sandwiched between upper and lower metal gratings, is proposed. This new design overcomes the incapability of existing reflectors to simultaneously optimize the reflectivity and reflection bandwidth. The reflection performance of the reflector was analyzed by the finite difference time domain method, and the number of metal layers, thickness of the polyimide film, length of the grating period, and grid width were optimized for achieving wide-band reflection and high reflectivity. Results show that this reflector has excellent reflection performance and large reflection bandwidth in the terahertz frequency band. In the 0-4.0-THz working band, the bandwidth with reflectivity above 0.98 can reach 2.04 THz. The proposed method provides a design scheme for high-performance reflectors for the development of terahertz broadband communication technology.