In this paper, an approach to achieve wide-angle beam scanning in the terahertz (THz) band with frequency sweeping was proposed based on a metasurface structure. To widen the scanning angular scope, a coherent enhancement mechanism for improving the mode competition efficiency of the high-order diffraction mode was first implemented. A systematic method was firstly developed for the design of practical frequency-controlled beam scanning quasi-optics in THz band, in which the optimization of scanning range, linearity, and the avoidance of beam blocking are comprehensively considered. The proposed concept and approach was verified by the measured wide-angle beam scanning at the 0.18-0.22THz frequency band, with a diffraction efficiency of the main lobe as high as 88 percent. The good performance of the device paves the way for potential applications in THz imaging, moving target detection, and communication.