The invention discloses a method and system for determining lightning shielding failure performance of multiple-circuit lines on the same tower, and belongs to the technical field of high voltage. Themethod comprises the steps of conducting a gap discharge test on the multi-circuit lines on the same tower to obtain typical electrode test data, and fitting the test data to obtain a strike distanceand a ground strike distance of the multi-circuit lines on the same tower; according to the strike distance and the ground strike distance of the multi-circuit lines on the same tower, obtaining exposed arc sections corresponding to each layer of cross arm wires and ground wires of the multi-circuit lines on the same tower and the projection lengths of the exposed arc sections on a lightning incident vertical plane; and obtaining the shielding failure flashover rate of each circuit line of the multi-circuit lines on the same tower, and determining the lightning shielding failure performance of the multi-circuit lines on the same tower according to the shielding failure flashover rate. According to the invention, simulation calculation can be carried out on the lightning shielding failuretrip-out rate of each circuit line in the multi-circuit transmission lines on the same tower, and theoretical support is provided for evaluating and optimizing the lightning protection design of the multi-circuit transmission lines on the same tower.