Based on Wudongde multi-terminal UHVDC demonstration project, the control strategy of multi-terminal hybrid UHVDC transmission system is researched to adapt to the characteristics of complex operation and high reliability requirements. The control mode and starting-up/shutting-down approach applied to multi-terminal hybrid UHVDC system are proposed, which could realize the matching of external characteristics of hybrid DC, two/three terminal stations starting-up/shutting-down, and the third terminal station online inputting/quitting. Also, the steady state control strategy of each terminal and multi-terminal coordinated control strategy are also studied, which could adapt to the complex operation of multi-terminal hybrid HVDC transmission system. Finally, the simulation results based on PSCAD/EMTDC show the validity of proposed control strategy, verifying the reliable and stable operation of multi-terminal hybrid UHVDC transmission system.