The technical scheme of a CLLC type modular input-series output-parallel DC transformer is presented, which can achieve voltage conversion and power transmission between high and low voltage of DC grid. CLLC series resonance transformer is adopted as submodule of DC transformer, which works at open loop mode, constant frequency mode and full resonance mode with high efficiency and constant voltage gain, and the input voltage and output current of each submodule are naturally equal. In order to prevent the large impact current, a soft start control strategy is proposed, in which the duty ratio D slowly increases from D_min to 0.5 by means of pulse modulation control. A 500 kW ±7.5 kV/800 V DC transformer prototype is built, the correctness and feasibility of the proposed topology and design method are verified by the experimental results.