In this Letter, a frequency-controlled wide-angle beam scanning antenna operating at 0.3 THz was proposed based on metasurface. The antenna formed by H-shape cells is periodical, and its’ scanning property is analysed by the Floquet theorem. To widen the scanning angular scope, higher order mode's diffraction wave was proposed. A systematic method was developed for the design of a practical frequency-controlled beam scanning antenna in THz band, in which the optimisation of scanning range, linearity and the avoidance of beam blocking are comprehensively considered. Additionally, a coherent enhancement mechanism for improving the mode competition efficiency of the high-order diffraction mode was implemented. The proposed antenna and optimisation method was verified by the measured wide-angle beam scanning at the frequency band 0.25–0.3 THz, the measured beam scanning range of the main lobe covers from −41.7° to −25.5° with a good linearity, which is much wider than that of the first-order mode. The diffraction efficiency of the main lobe measured is 74% on average, which indicates the remarkable inhibition of the unwanted diffraction.