The invention discloses a parameter design method for a smoothing reactor of a looped-network flexible DC power transmission system, and the method comprises the steps: obtaining an expected operation state when a DC side of the flexible DC power transmission system has a unipolar grounding fault; obtaining all limiting conditions of the parameter design of the smoothing reactor based on the expected operation state; jointing all limiting conditions after a fault current of the DC side is completed removed, and obtaining the value range of the parameters of the smoothing reactor of the flexible DC power transmission system. Compared with an existing method for the determining of a low threshold value, the method provided by the invention proposes the clearer limiting conditions for the calculation of the fault current, thereby further reducing the size of the range of the parameters needed by the smoothing reactor, and enabling the calculation result to be clearer.