This paper presents a single-layer transmission-type metasurface based on geometric phase, which enables the generation of broadband Terahertz vortex waves through the design and arrangement of metasurface unit structures. The metasurface unit consists of a top layer with dual C-shaped metal rings and a bottom dielectric layer. By utilizing the geometric phase principle and rotating the metal structures, the phase of cross-polarized transmitted light can be controlled while maintaining the same transmission coefficient over a wide frequency range. By encoding the unit structures with rotation, an encoded metasurface is formed capable of generating vortex beams with different topological charges in a wide frequency band. Simulation results demonstrate the generation of vortex beams with topological charges of l=±1 in the frequency range of 1.38-1.92 THz, with a maximum cross-polarization transmission efficiency of 23%. close to the theoretical limit of a single-layer structure.