The invention discloses an efficient 220GHz third harmonic mixer using a single tube core, which comprises a local oscillator input standard rectangular waveguide, a radio frequency input standard rectangular waveguide, a quartz substrate and a suspended microstrip-microstrip hybrid transmission circuit. The local oscillator input standard rectangular waveguide is directly connected with the suspended microstrip-microstrip hybrid transmission circuit, the RF input standard rectangular waveguide and the suspended microstrip-microstrip hybrid transmission circuit are in transition connection, and the suspended microstrip-microstrip hybrid transmission circuit is arranged on the quartz substrate, the quartz substrate is placed in the shielding cavity, wherein the suspended microstrip-microstrip hybrid transmission circuit comprises a local oscillator matching waveguide, a matching and reflecting branch, a radio frequency short-circuit branch, a single-die diode pair, a ground metal, a local oscillator matching branch, a radio frequency waveguide microstrip transition, and a radio frequency low-pass filter. The frequency conversion loss is reduced, and the influence of the feed loss onthe frequency conversion loss is small, so that the radio frequency signal and the local oscillator signal can participate in mixing effectively without leaking each other.