The invention discloses a method for designing the parameter of a direct current reactor suitable for a modular multi-level converter. The method includes the steps that the maximal instantaneous current value Ilim allowed by a bridge arm is determined according to the parameter of a converter valve; the maximal current value Ifault of the bridge arm before a system is locked is calculated when a direct current polar line short-circuit fault happens; the maximal instantaneous current value Ilim allowed by the bridge arm is compared with the maximal current value Ifault of the bridge arm before the system is locked, and the lower limit value Llim of the inductance of the direct current reactor is calculated to serve as the final design value of the inductance of the direct current reactor. The method has the advantages that the inductance value of the direct current reactor is designed through the maximal failure current limiting value condition borne by the bridge arm, and meanwhile the influence on failure currents by alternating current feed-in and capacitor discharge is considered. The calculation method is simple, and calculation results are more accurate.