We demonstrate that when a charged particle moves on top of a metal-slot metasurface consisting of metallic slot resonators, strong Smith-Purcell electromagnetic (EM) radiation can be produced at resonant frequency. By adjusting the period of the metasurface, the resonant (or working) frequency can be tuned from gigahertz to terahertz and infrared regions. Since the EM field is localized in the slots rather than at the metal surface, the metasurfaces are found to exhibit a very low absorption loss ratio (<1%) in low working frequencies (<1 THz). Although it becomes larger in high frequencies (>1 THz), the loss ratio remains relatively low (<12%). In addition, a nonlinear relationship is also uncovered between the resonant frequency and the reciprocal of the period. Our results could benefit the construction of efficient, compact terahertz, and infrared free-electron light sources.