Modular solid state transformer(SST) has prevailed in recent years, due to its feasibility for power management. Generally, an SST uses power semiconductors on medium voltage gird side to support the grid voltage, and it uses medium frequency transformers to replace the bulky line-frequency transformer. Therefore, a competitive lifespan of an SST compared with conventional power conversion system is a key topic in this area. Thermal control plays an important role in lifespan management of power semiconductors, which is the key components in SSTs. Thus, this paper proposes an active thermal control strategy for Cascaded H-Bridge(CHB) in SSTs to increase lifespan of the rectifier stage. Employed by the proposed strategy, simulated circuit shows that thermal stress on power semiconductors in CHBs is effectively balanced, and the maximum junction temperature can be reduced by 10.2%, which leads to a longer lifespan of key components.