The DC-DC converter is widely used in electric vehicle charging and the photovoltaic power generation system. How to adapt to input/output voltage changes over a wide range, and realize the wide gain and maximum transmission efficiency have attracted attention from academia and industry. Especially, since the LLC and LLC_LC as well as the LLCLC resonant converter have characteristics such as high-power density and low electromagnetic interference, there are shortcomings including difficulty in designing magnetic components and resonant network parameters. These cause instability for the converter output, and it is difficult to meet the requirements for practical application. Thus a wide gain and high efficiency resonant DC-DC converter technology is proposed. The fundamental principle of the resonant DC-DC converter is summarized. Then research progress, both domestic and abroad, for topology and modulation strategy, is examined. The paper elaborates the application requirement for a wide gain and high efficiency resonant DC-DC converter. Then the topology and modulation strategy for both LLC_LC and LLCLC with multi-mode PWM voltage- multiplying rectifying converter are analyzed. Through simulation and experimental verification, the effectiveness of the topology and modulation strategy for two wide gain and high efficiency resonant DC-DC converters is verified, to show a maximum output voltage range of 1~6.2, and a conversion efficiency of 96.1%. This shows good prospects for application.