To improve the reliability of the MMC converter, the heat balance control is introduced on the basis of the traditional mean voltage control. By controlling the switching process of the sub module to reduce the temperature fluctuation of the sub module, the reliability of the MMC converter is improved. To estimate the temperature of the sub module, the temperature calculation method based on the Forster thermal network model of the IGBT module is proposed. In the process of voltage balance and heat balance control, it selects the submodules that need to change the state. The sub module temperature can be reduced by reducing the unnecessary submodule switching action. The control method is validated by using Matlab/Simulink simulation platform, and the results of the test are compared with the temperature fluctuation and the uniform pressure effect of the traditional pressure control. The results show that the control method can effectively reduce the temperature fluctuation of the device and improve the reliability of the converter, and has good practicability. This work is supported by Natural Science Foundation of Hunan Province (No. 2018JJ2045).