The invention provides a terahertz wave tunable mode converter, which comprises a hollow cylindrical metal inner-wall waveguide formed by rectangular periodically fluctuating structures. According to the invention, resonance interaction among different transverse standing wave modes in the waveguide is utilized to realize conversion of terahertz waves from a fundamental mode to a single high-order mode. Furthermore, tuning for output frequency of the terahertz wave mode converter can be realized through changing the thickness of a metal coating in the rectangular periodically fluctuating pipe wall, and the tunable mode converter can realize that the output center frequency is linearly tunable in broadband. The terahertz wave tunable mode converter is reasonable in design, has the characteristics of simple structure, wide tunable range, low loss, high conversion efficiency, convenient integration and the like, can acquire wide applications in fields which have special requirements for terahertz wave mode field distribution and the operating frequency, and provides a new choice for designing and manufacturing functional devices in a terahertz wave application system.