A miniaturized double-sided anti-metal UHF RFID tag with a spiral loop structure is proposed. The tag antenna comprises three metallic layers: a middle radiation patch, a lower ground plane, and an upper ground plane, separated by two layers of 1 mm-thick foam substrates. The middle radiation patch consists of an external ring patch and a central spiral loop patch, with the tag chip positioned between them. The middle radiation patch and the upper ground plane together generate effective radiation when the lower ground plane is mounted on the metallic plate. Conversely, the middle radiation patch and the lower ground plane act as the radiating pair when the upper ground plane is mounted on the metallic plate. This design enables the proposed antenna to achieve both double-sided anti-metal performance and miniaturization. Theoretical analysis and simulation results confirm that adjusting the length of the spiral loop patch in the middle layer effectively tunes the antenna′s inductive reactance, which enables conjugate impedance matching between the antenna and the chip. The tag antenna has dimensions of 35 mm×20 mm×2.15 mm. Simulation and experimental results demonstrate that the maximum power transfer coefficient between the tag antenna and the chip exceeds 98% for both sides. When the two ground planes of the tag antenna are individually mounted on a 200 mm×200 mm metallic plate under an effective isotropic radiated power (EIRP) of 3.28 W, within the 902~928 MHz frequency range, the read distances reach 7.5 and 7.6 m, respectively. The minimum sensitivity is -14 dBm for both sides. The proposed tag offers advantages such as compact size, consistent double-sided anti-metal performance, and long read distance. It effectively enhance the identification capability and stability of suspended tags in metallic scenarios, providing an efficient solution for RFID tag applications in complex metallic scenarios.