A grating-resonator composite structure was investigated. The output characteristics of Smith-Purcell radiation in this grating-resonator composite structure were studied by optical theoretic analysis and particle-in-cell simulation method. The results show that tunable coherent Smith-Purcell radiation at Terahertz wavelengths can be generated by this novel structure. This novel grating-resonator composite structure has the following advantages: it can reflect all radiation with an emission angle and random azimuthal angles which backs onto the electron beam with same phase and cause the electrons to be modulated.