The present invention relates to a power station lightning arrester nominal discharge current determination method and system. The method comprises: obtaining the operation voltage grade and the electrical topological structure of a power station to establish an ATP-EMTP model corresponding to the power station; obtaining pre-defined at least two thunder influencing factors, obtaining a preset value corresponding to each thunder influencing factor, inputting the at least two thunder influencing factors and the corresponding preset values into the ATP-EMTP model, and simulating the thunder discharge current of a power station lightning arrester in the condition of different invasion routes through the ATP-EMTP model; and determining the nominal discharge current of the lightning arrester according to the thunder discharge current of the power station lightning arrester in the condition of different invasion routes, wherein the nominal discharge current is not less than the thunder discharge current in the condition of any one invasion route. The power station lightning arrester nominal discharge current determination method and system can effectively determine the nominal discharge current of the power station lightning arrester to allow the power station lightning arrester to satisfy actual thunder discharging condition in various conditions.