To achieve the dual-carbon goal, a large-scale wind power photovoltaic base is being built in the desert, barren and gobi areas. Aiming at the problems of large scale, wide area, lack of conventional power supply support and distribution feature of electric energy and load center at Shagehuang renewable energy base, a hybrid DC transmission system combining the benefit of line commutated converter (LCC) and modular multilevel converter(MMC) is proposed. Main feature of the system is that rectifier side is consisted of LCC and MMC in series, and inverter side is consisted of three parallel MMC and LCC connected in series with different AC systems. According to the characteristics of the system topology, the mathematical model is built, and the control strategy of the system is proposed under fault state and steady state. Due to the complex coupling characteristics between the inverter side converter stations, the interaction between the converter stations is further analyzed. The system simulation model is built by PSCAD / EMTDC simulation platform to test the effectiveness of the theoretical analysis and the proposed control strategy.