Hybrid HVDC can take full advantage of the thyristor converter and voltage source converter,and it has a good development prospects.In order to analysis the control strategy and operating characteristic of hybrid DC grid,a four terminal bipolar's hybrid grid structure is established,whose rectifier side is modular multilevel converter and inverter side is thyristor converter,the mathematical model is derived and corresponding control strategy is designed.The start-up process and operating characteristic of hybrid DC grid are simulated and analyzed in PSCAD/EMTDC,which demonstrates the effectiveness of the proposed control strategy.