Designs an AVR-based constant temperature controlling system which collecting temperature by DS18B20 sensor, monitoring temperature by PN junction diode and controlling electrothermal wire heating by MOS transistor switch. Introduces the selection of system hardware circuit and the realization of system software, including the temperature signal collection, PID control and display, etc. The experimental results show that the proposed system has controlling accuracy of ±0.1℃, temperature resolution of 0.06℃ and well stability.