The invention discloses a method for calculating 50% discharge voltage under tower clearance switching impulse voltage, and is applicable to an extra-high voltage direct current transmission system. The method comprises the following steps: acquiring a plurality of first data points on a first clearance path between a wire and a tower in the extra-high voltage direct current transmission system, and acquiring the potential of each of the first data points; acquiring a plurality of second data points on a second clearance path between an analog wire and a vertical bar in a tower clearance discharge model, and acquiring the potential of each second data point at different heights of the vertical bar; calculating a determination coefficient between the potential distribution of wire-tower andthe potential distribution of the analog wire-vertical bar according to the potentials of the first data points and the potentials of the second data points; and calculating the 50% discharge voltageunder the switching impulse voltage of the tower clearance discharge model. The invention further discloses a system for calculating the 50% discharge voltage under the tower clearance switching impulse voltage. The method for calculating the 50% discharge voltage under the tower clearance switching impulse voltage provided by the invention can effectively simulate the process of wire-tower clearance discharge voltage, save time and reduce costs.