The invention discloses a device and a method for testing the armor loss of an alternating current submarine power cable, and relates to a loss testing device and a loss testing method. At present, the loss testing device has higher requirements on the through-current capability of the secondary side of a transformer, transformer loss is involved, and loss testing errors are greater. The alternating current submarine power cable comprises a cable core, a metal sheath wrapping the cable core and an armor. The device for testing the armor loss of the alternating current submarine power cable is characterized by comprising transformers and a tester for detecting current and voltage, wherein the tested submarine power cable is arranged on the secondary sides of the transformers in a penetration way, and the front end and the tail end of the tested submarine power cable are connected; and the section, which penetrates through the transformers, of the tested submarine power cable forms secondary windings of the transformers, and the other part of the tested submarine power cable is a load. According to the technical scheme, loss errors of the transformer are eliminated, data is more authentic and reliable, and requirements on test equipment are lower.