Ultra-broadband harmonic mixers generally use LO-IF duplexers to extract IF signals from the LO circuit. However, when the LO frequency and the IF frequency are similar or overlap, it is difficult to achieve high LO-IF isolation. In this paper, RF-IF duplexer is used to replace the LO-IF duplexer, different from the common harmonic mixer structure, and IF signal is extracted from the RF circuit. A 30~110 GHz fourth harmonic mixer chip is designed and tested. After the four harmonic mixer testing, RF sweep frequency range 30~110 GHz, frequency conversion loss is less than 25 dB at IF frequency 1 GHz. In DC-15 GHz, the isolation between the LO and the IF ports is up to 30 dB. When the local vibration is fixed, the frequency conversion loss of IF sweep frequency DC-7 GHz is less than 28 dB. Therefore, this design can effectively isolate LO and IF signals with similar frequencies, and it is possible to broaden the IF broadband.