Abstract: The displacement current and space electromagnetic waves induced by the rapid changes in the electric field at the rising and falling edges of the square wave can cause considerable interference to the measurement of partial discharge (PD). Therefore, we designed and verified a PD detection method based on super high frequency (SHF) signal acquisition and down-mixing technology, which is appliable to the insulation system under square wave. The system performance tests verify that the SHF mixing method can suppress the interference from the square wave effectively. The mixed-signal retains the SHF signal amplitude and phase while lowering the frequency. It is also verified that the designed system can detect the PD with an apparent discharge level of about 20 pC. The experimental results of PD detection show that, compared with pulsed current method and UHF method, the SHF mixing method has better performance for the PD with a faster discharge rate occurring in the material with higher dielectric strength. Furthermore, the PD experiment was performed on epoxy resin and potting silicone under square wave, verifying the feasibility of SHF-mixing method.