In this paper, a dc-dc resonant converter is proposed for high-power(HP) and medium-voltage(MV) application, which adopts series-connected half-bridge sub-modules to build the inverter unit of the conventional LLC converter. And the gate signals of each module have a tiny phase shift angle to reduce the voltage change rate of the converter and simplify the insulation design difficulty of magnetic component. Open-loop control method is employed to equalize the voltage of the sub-module capacitances, thus simplifying control. This paper analyzes the operating principle and voltage gain of the converter and determine the control strategy. Finally, the topology is verified by building a 21kW, 3kV to 500V simulation model and an experimental prototype with an efficiency of 96.77% under the condition of 3kV/12kW.