In this paper, we design a temperature sensing system based on a DSP chip TMS320F2812 for a real-time PCR instrument. In this system, we design a 4-wire temperature sensing module with a thermal resistor PT100 to measure the temperature and convert it into a voltage signal. Then the voltage signal is digitalized by the ADC module. At last, the data are transferred to a PC through the serial port. The experimental results show that the linearity of the system is 0.99 with the standard deviation of 0.0419.