A master control method for a series multi-terminal direct current (MTDC) system and elements thereof. The method comprises: choosing one terminal as a current setting terminal (CST) and defining other terminals as voltage setting terminals (VST) (302); configuring a current reference of the series MTDC system as the input of the CST converter, generating current values for each VST converter (304) based on the current reference and different margins thereof respectively and making the minimum value of current reference in the rectifier side larger than the maximum value of current reference in the inverter side (306). The method for series MTDC system and elements thereof can regulate and optimize the active power and reactive power, reduce the power losses, and control the distribution of the reactive power.