For pre-research on ice cloud detection equipment, the design and test of a low conversion loss （CL） 4^th-harmonic mixer working at the central frequency of 670 GHz based on an anti-parallel Schottky diode are presented. In order to improve the mixing efficiency, a two-stage local-oscillator （LO） compact-microstrip-resonate-cell （CMRC） low-pass-filter （LPF） is utilized to suppress the second mixing item （）, the third harmonic of LO signal（）, and RF signal. The LO frequency is only one-quarter of the RF frequency which significantly reduces the complexity and cost of the LO chain. The measurement result shows that the Single-Side-Band （SSB） conversion loss is between 16.7 dB to 22.1 dB from 640 GHz to 700 GHz at room temperature. The minimum conversion loss is 16.8 dB at the RF frequency of 665 GHz.