The invention provides a low-temperature back-field magnet and a high-temperature superconductive unit alternating-current loss measuring device. The low-temperature back-field magnet comprises a non-magnetic-conductive spacing layer, a left low-temperature superconductive coil magnetic body and a right low-temperature superconductive coil magnetic body, wherein the center of the non-magnetic-conductive spacing layer is provided with a through hole which is through along the radial direction; the left low-temperature superconductive coil magnetic body comprises M coil layers Ri sequentially sleeved coaxially; the coil layers Ri of the left low-temperature superconductive coil magnetic body are correspondingly connected in series with coil layers Li of the right low-temperature superconductive coil magnetic body to form a merged coil layer Wi, wherein the merged coil layer Wi and a merged coil layer W(i+1) are sequentially connected in series to form a whole coil; and in the using process, the whole coil is connected to the two sides of a direct-current power source, and a back magnetic field with preset intensity is generated in the through hole. According to the low-temperature back-field magnet, magnetic field density is high, and magnetic field intensity adjustment is convenient. Through the high-temperature superconductive unit alternating-current loss measuring device, measurement is convenient, flexible, fast and accurate.