Positive and negative bipolar hybrid multi-terminal DC transmission system has the advantages of low cost, simple control and rich engineering experience of LCC-HVDC,and has the advantages of independent control of active power and reactive power and isolated island power supply of VSC-HVDC. At the same time, the system wiring and operation mode are flexible, so it has broad application prospects in the field of DC transmission in China. Firstly, the system topology and converter topology of this system are introduced. Secondly, the mathematical model of LCC converter and MMC inverter are deduced in detail, and its related control strategy is designed based on this. Thirdly, for this system, the coordinated control strategies between the converter stations under steady-state and single-valve fault emergency exit conditions are designed. Finally, in the PSCAD/EMTDC environment, the electromagnetic transient simulation model was built with reference to the relevant parameters of the Wudongde engineering system. The correctness of the relevant control strategy and the flexibility and reliability of the system operation were verified by simulation.