ABSTRACT Heat time constant is an important technology parameter of thermal resistance. Fever of the main body of a book having been introduced time constant import and expression. In this paper, the principle and method of the test have given; they are ease of use, stable and reliable, high universality, and other advantages. MCU is to dominate the chip, a method and system for measuring time constant of thermal sensor was put forward. The system heated electrically the thermal sensor firstly, and then made it cool in the medium naturally