A novel type of slow-wave structure for Smith-Purcell device called dielectric loaded metal grating, is proposed in this article. The “hot” dispersion align of the structure is obtained by using the eigen-function method and single-mode approximation. The first-and second-order growth rate of beam-wave interaction are obtained at the synchronization point. The effects of grating groove width and depth on dispersion characteristic are analyzed, and the influences of electron beam parameters and distance between electron beam and grating surface on growth rate characteristic are also studied. The results show that dielectric-loaded metal grating can effectively weaken the structure dispersion, and that with the increases of relative dielectric permittivity, groove width and depth, the dispersion curve becomes flatter and moves toward low frequency. When the electron beam voltage or current changes, the first-order growth rate curve can only roughly describe the change trend, while the second-order growth rate can accurately show the change values. The simulation of the structure is performed by using two-dimensional particle-in-cell code MAGIC, and the simulation results accord well with the theoretical results.