This paper focuses mainly on UHF passive RFID tag design and presents some design consideration about several key circuits of the RFID tag. The author begins from the analysis of the basic structure of the UHF RFID tag. Four kinds of power supply recovery circuits are introduced first. Then the author gives us a solution about using Schottky diode to build a voltage doubler with standard CMOS process. About power supply regulator, this paper presents \'cascade\' and \'parallel\' two kinds of voltage regulators. A new current sink is designed, which is aimed at ASK envelope demodulation. At last, this paper introduces the design of the power-on-reset circuit.