As the key equipment for interconnection between DC power lines with different voltage levels, high-voltage DC/DC converters play an important role. However, the traditional high-voltage DC/DC converters show defects such as the low voltage gains, large volumes and weights, etc. This paper proposes a resonant modular high-gain step-up DC/DC converter (RMHGDC), which operates in resonant mode between submodule capacitors and the arm inductor,features with series-connected submodules for obtaining higher voltage gain. The basic operational principles are analyzed firstly. Based on deduction of the mathematical relation of key parameters of RMHGDC, the high gain control strategy is proposed and studied in detail. An experimental prototype is constructed to verify the feasibilities and effectiveness of the RMHGDC and the proposed control strategies.