A hybrid direct current transformer (DCT) with modular multilevel structure and series-connected switches experiences current spike in sub-module branch when using original control method. The existence of current spike causes extra current stress on components. Based on the operating principle and current waveforms,an optimization modulation method is proposed,which is based on the phase shift of medium-voltage side full-bridge. The current stress of series-connected switches branch is analyzed,combined with limitation of phase shift angle for operation safety,the algorithm for the optimum phase shift angle is determined,and the optimized control strategy is established. This strategy is independent of the original power control loop,so it does not change the power transmission status and interfere with power control,which makes it easy to be put into practical application. The effectiveness of current stress reduction is verified by simulation and prototype test,and a certain improvement of efficiency is shown by the results of prototype test. Based on the analysis of operation principle and simulation and test results,the optimal control strategy is proved to have positive significance for the safe operation of equipment and selection of components.