The invention discloses a comparator for AC measurement. The comparator comprises a sensing head, a preamplifier, a power expander and a standard resistor. The sensing head comprises a ring detectingcore, a detecting winding and an inner magnetic shielding core, inner and outer secondary windings, and an outer magnetic shielding core. The detecting winding is wound around the ring detecting corewhich is placed in the annular cavity of the inner magnetic shielding core. The inner secondary winding is wound around the inner magnetic shield core. The inner magnetic shield core around which theinner secondary winding is wound is integrally disposed in the annular cavity of the outer magnetic shield core. The outer secondary winding is wound around the outer magnetic shield core. The inner magnetic shielding core, the inner and outer secondary windings and the outer magnetic shielding core form an AC magnetic potential balance system based on the principle of a two-stage current transformer, thereby achieving high measuring precision. The dual magnetic shield core structure formed by the inner and outer magnetic shielding cores has a good in anti-interference capability.