Abstract: Due to a DC component in the arm current, the junction temperature distribution of the four power devices within one submodule (SM) in a modular multilevel converter (MMC) becomes unbalanced. The unbalanced distribution of thermal stress within the SM results in significant differences in lifespans among the switching devices, which can seriously threaten the safe and reliable operation of the converter. To solve the problem, a thermal balancing control strategy inside the SM is proposed, which considers the correlation between the operational state of the SM and the junction temperature of the switching device. The proposed strategy reduces the junction temperature difference of switching devices by adjusting the switching function of SMs while improving the reliability of the system without affecting the harmonic characteristics of the MMC's output current. This paper analyzes the mechanism of thermal imbalance inside the SM. The thermos-electric coupling model is established to study the relationship between the state of SM and the junction temperature of switching devices. The thermal balancing is realized by controlling the average input time of SM. Finally, the effectiveness of the proposed strategy is verified through thermoelectric co-simulation using Matlab/PLCES, as well as the experimentation on the mission profile emulation-based platform for SMs.