DC transformer is the core equipment for realizing the DC collection and transmission of new energy. A DC transformer based on a hybrid combination of current source converter and modular multilevel converter (CSC-MMC) can achieve high controllable commutation and efficient power transfer. This article, which is based on the CSC dual degree of freedom fundamental frequency modulation method, establishes a mathematical model of CSC-MMC DC transformer, explores its power transfer relationship, and proposes a steady-state control strategy for DC transformer. Finally, the effectiveness of the proposed control strategy was verified through PSCAD/EMTDC.