How to measure temperature realtime in high precision and monitor temperature has important significance. A fourwire high precision temperature measuring and monitoring system using PT100 was designed. After exited by a constant current source using integrated components OP484 and TL431, the thermal resistor output a voltage signal. After amplification by a signal modulation and amplifying circuit, the signal was converted to digital signal using TLC2574 AD converter. Microcontroller ATmega128 accepted and interpretated commands from PC through an integrated RS232 interface circuit MAX232, as to control start and stop operation, sampling time, precision of the data acquisition and so on. The system eliminates the measuring error due to connecting wires of the thermal resistor, and has the characterstics of high precision and strong ability of humancomputer interaction. The circuit system design is simple, practical, stable and reliable.