A method to Design the DC reactor is proposed for modular multilevel converter based high voltage direct current(MMCHVDC). Firstly, the maximal current value allowed in an arm was acquired base on the parameters of the valve. Secondly, the maximal value of fault current when a DC pole-to-pole fault was happening was calculated. If the maximal value of fault current was smaller than or equal to the maximal current value allowed in an arm, the DC reactor was useless. If the maximal value of fault current is larger than the maximal current value allowed in an arm, the DC reactor was needed. Lastly, an inequation was obtained based on that the maximal value of DC line-to-line fault current should be smaller than the maximal current value allowed in an arm to calculate the lower limit value of DC reactor. And the lower limit value was the design value of DC reactor. A two-terminal MMCHVDC transmission system is constructed in PSCAD/EMTDC, and simulation results show that the method mentioned above is correct.