The HVDC Flexible system is a novel DC transmission technology based on voltage source converters（VSCs）and insulated gate bipolar transistors（IGBTs）.With the drawbacks of the conventional HVDC system overcome，the new system has a bright application future.A description is made of the modular multilevel converter（MMC）and its application principle in the HVDC Flexible system.The differences between the MMC based valve base controllers of HVDC Flexible and traditional valve base electronics are indicated.And the functions and features of the valve base controller are studied.A49-level dynamic simulation experimental platform is set up to perform functional verification for researches on the HVDC Flexible system to refer to.