The invention provides an AC filter differential protection method based on a high-voltage side current effective value. The method comprises the following steps that step one, high-voltage side current and low-voltage side current of an AC filter are sampled; step two, 50HZ filtering is performed on the sampled high-voltage side current and low-voltage side current, differential current of high-voltage side and low-voltage side current is calculated by adopting a 50HZ component and is compared with a differential current fixed value; step three, a high-voltage side current effective value is calculated and compared with an effective value fixed value; and step four, when differential current of high-voltage side and low-voltage side current is greater than the differential current fixed value and the high-voltage side current effective value is greater than the effective value fixed value, actuation of differential protection is outputted without time delay, a high-voltage side switch is tripped off, and differential protection does not actuate under other conditions. Distinguishing is performed by adopting different characteristics of high-voltage side current when the AC filter withdraws and fails to work so that reliable actuation protection in a local grounding fault can be guaranteed and no actuation in the withdrawing process of the AC filter can be reliably guaranteed.