Quantum cascade laser is a newly developed important medium and far infrared laser source. In view of the important parameter of energy band electron temperature in the research and design of quantum cascade lasers, based on the relationship between electron kinetic energy and temperature and Fermi Golden Rule, this paper optimizes the rate equation so that it can calculate the subband electron temperature, thus achieving a more accurate solution of the rate equation. The calculation results show that compared with the existing kinetic energy balance method, this method describes the process of electron temperature change in the energy band in detail, and there is no need to use the optimization algorithm for a solution. When different initial temperatures are selected, the electron temperature of each energy level can be solved by self-consistent solution, and the convergence value with good consistency can be obtained. The results show that the deviation of the convergence value of electron temperature from the mean value is less than 8%, and the deviation of scattering rate is less than 1.6%. This study provides a new method for the design and research of quantum cascade lasers.