Disclosed is a distributed grounding electrode design method for a high-voltage direct-current transmission system. The distributed grounding electrode design method includes the steps of 1), preliminarily selecting the scope for grounding electrodes to be established according to planning and land expropriation, and determining the best position for the grounding electrodes to be established through a global optimal position mutation particle swam optimization algorithm; 2), determining the biggest potential rise of sub-electrodes according to temperature rise of the grounding electrodes and checking step potential; 3), connecting all the grounding electrodes with newly established sub-electrodes through an overhead line; 4), establishing a unified model for the distributed grounding electrodes and direct-current distribution of a alternating-current network; 5), according to the solved direct-current distribution of the alternating-current network, predicting risks of direct-current magnetic bias occurring to transformers of the alternating-current network; 6), for the transformers with the risks of direct-current magnetic bias in prediction, taking measures to restrain or eliminate negative influence brought by the problem of the direct-current magnetic bias. By the distributed grounding electrode design method, the risks of the direct-current magnetic bias can be reduced, and the degree of the direct-current magnetic bias after establishment of the grounding electrode can be estimated, so that damage brought by the direct-current magnetic bias can be reduced.