We presented an ultra-thin optical waveguide which can be used as slow wave structure for Terahertz (THz) TWT in this paper. This ultra-thin optical waveguide has large dimensions, which can be fabricated by microelectronic fabrication processing. The simulation results show that when the height of waveguide less than 45μm, this waveguide has abnormal limitation of mode- filed, most of the confined power is transmitted in air, which can be used to interaction with electronic beam. Based on this, this waveguide can be designed as meander line slow wave structure (SWS). The dispersion and pierce impedance were simulated and the simulation results show that this structure is suitable for TWT design.