The invention discloses a method for diagnosing an impact on a metallic oxide arrester electrification test from inter-phase interference. The method includes: acquiring instantaneous voltage values at various sampling times from leads on high-voltages sides of an A-phase metallic oxide arrester and a C-phase metallic oxide arrester, calculating phase angles of instantaneous voltage via the Fourier algorithm, acquiring instantaneous leakage current values at various sampling times from grounding down leads of the A-phase metallic oxide arrester and the C-phase metallic oxide arrester, separating resistive current from instantaneous leakage current, calculating a phase angle of the resistive current via the Fourier algorithm, calculating an interference coefficient, and diagnosing by comparing the interference coefficient with a setting valve. By the method, the impact on the metallic oxide arrester electrification test from the inter-phase interference is accurately diagnosed, if any inter-phase interference leads a metallic oxide arrester electrification test result to be abnormal, the diagnosing result can provide powerful reference to testing workers for accurately analyzing insulating performance of the metallic oxide arresters, and testing workers' judgment error caused by the abnormal testing result due to the inter-phase interference can be effectively avoided.