In a sliding-IF receiver, the phase shift between two local oscillators (LOs) affects the down-conversion gain of multiple harmonics. This paper analyzes the impact of the phase shift between two LOs on the down-conversion gain of three key harmonics more comprehensively. Firstly, a detailed mathematical analysis is provided for both quadrature and non-quadrature demodulation methods. The impact of interference at frequencies other than the image frequency was also focused on and analyzed. Then, based on the results of the mathematical analysis, it is determined that in quadrature and non-quadrature demodulation, a shift of one-eighth and one-quarter of the period of the first LO between the two LOs’ the rising edges offers better interference suppression effects, respectively. Finally, to verify the correctness of the theoretical analysis, a printed-circuit-board-level sliding-IF passive mixer is designed. The measurement results are found to be consistent with the theoretical analysis.