The invention relates to a broadband millimeter wave second harmonic mixer, and belongs to the field of radar radio frequency solid-state devices. The mixer comprises a local oscillator input bending waveguide (1), a local oscillator input height reduction waveguide (2), a local oscillator input short circuit surface (3), a radio frequency input height reduction waveguide (4), a radio frequency input short circuit surface (5), a radio frequency direct current grounding (6), a radio frequency waveguide microstrip probe transition (7), a diode matching (8), a Schottky plane diode (9), a local oscillator matching filter (10), a local oscillator waveguide microstrip probe transition (11), an intermediate frequency low-pass filter (12), an intermediate frequency output (13), and a mixer circuit board (14). The device is simple and exquisite in structure, high in isolation degree, wide in working frequency band and low in frequency conversion loss. The problem of low frequency conversion loss realization in a wide frequency band of 84 GHz-94 GHz is effectively solved, the realization of a W-band high-performance and low-cost receiver is ensured, and the requirement of a high-performance test instrument is met.