Recent research progresses are summarized about the High Voltage Direct Current(HVDC) transmission system in control methods and modulation of HVDC transmission system,coordinated control of multi-infeed HVDC transmission system and AC-DC system.Some outsanding problems in current research are pointed out on such sides as system modeling,studying object,aim of coordinated control and engineering application.Based on the requirements of real systems and some latest research achievements on control technology and measure methods,some new research thoughts about HVDC transmission system for some problems of assessment and optimal combination of current control strategies,application of intelligent control methods and enhancement of coordinated control are proposed.