The Ultra High Frequency (UHF) passive Radio Frequency Identification (RFID) tag is becoming a research hotspot because of its obvious advantages as large working range, small antanna size and so on. The performance of the RF interface directly decides that of the RFID tag. In this paper, the principle of RFID tag power supply is expounded and a RF interface for UHF long distance passive RFID tag is designed, including power generation circuit, regulator and demodulator. This design solved the problems of remote power supply and information receiving of UHF passive RFID tag. The RF interface circuits are taped out in UMC with 0.18μm mixed signal process. The performance of the RF interface is proved by measurement to satisfy the requirements of the UHF long distance passive RFID tag.