DC transformer is one of the key devices connecting different DC voltage levels and building DC power interconnection. In order to realize the simple, reliable, efficient and low-cost DC transformer, a high-power high-step-up ratio DC solid-state transformer based on medium-frequency inversion is proposed in this paper. The DC transformer consists of parallel inverters module, medium-frequency 400 Hz 24-pulse phase-shift boosting transformer, three-phase diode rectified bridge and high-voltage filter circuit. The detailed design and modeling analysis of topology structure were performed and a hybrid control scheme based on active-power-master-slave and voltage-droop is proposed. Finally, simulation is built through Matlab/Simulink and then the low-power experimental platform is built for experimental verification. Both simulation and experimental results verified the correctness of topology and control scheme.