1. A hetero-integrated terahertz low-noise miniaturized image frequency rejection transceiver front-end, comprising an intermediate frequency circuit and a terahertz circuit arranged up and down, wherein the terahertz circuit comprises a local oscillator frequency tripler, a 135° 3 dB filter coupler, a first quartz hetero-integrated subharmonic mixer, a second quartz hetero-integrated subharmonic mixer, and a radio frequency waveguide power divider; the 135° 3 dB filter coupler comprises an input unit, a first output unit, a second output unit, and an isolation unit each of which is composed of a resonant cavity and a waveguide, and the resonant cavities of the input unit, the first output unit, the isolation unit, and the second output unit are sequentially coupled through resonant grooves to form a ring structure; a cavity length of the resonant cavity of the input unit is twice that of the resonant cavities of the other three units, and an electrical length of the waveguide of the first output unit is 45° different from that of the waveguide of the second output unit;