Aiming at the requirements of miniaturization and low profile of UHF RFID metal-mountable tag, this paper proposes a metal-mountable UHF RFID tag antenna with a size of 50 mm×20 mm×0.9 mm. The design uses CST MWS software for modeling and simulation. The effects of the changes of the size of the inset-feed structure and the size of the rectangular slot on the input impedance of the tag antenna are analyzed, and the corresponding parameters are adjusted to realize conjugate impedance matching between tag antenna and microchip. The measured results show that the input impedance of the tag antenna and the chip impedance are well matched. The tag antenna has a maximum measured read range of 4.3 m at 910 MHz, and has a small size and low profile. It can be applied to metal scenes in various fields such as logistics, medical treatment and retail.