A terahertz wave fast modulator based on coplanar waveguide combining with transistor is disclosed. The terahertz waves are inputted through a straight waveguide structure, and then are coupled through a probe structure onto a core part of the present invention, which includes a suspended coplanar waveguide structure and a modulation unit with high electron mobility transistor, wherein the suspended coplanar waveguide structure is formed by three metal wires and a semiconductor substrate; and the modulation unit with high electron mobility transistor is located between adjacent metal transmission strips of the coplanar waveguide structure. Transmission characteristics of the terahertz waves in the coplanar waveguide structure are changed through the switching on/off of the modulation unit, so as to fast modulate the amplitudes and phases of the terahertz waves, and finally the modulated terahertz waves are transmitted through a probe—waveguide structure.