According to the characteristics of the action of the lightning electromagnetic field on overhead lines, the relation formula is devised between the position of any given strike point and the lightninginduced overvoltage along an overhead line by mathematics models. The influences on lightninginduced overvoltage are analyzed of such factors as the height of an overhead line, the distance between overhead line and strike point, the height of an object beside the overhead line, the length of a raising leader (return stroke), etc. The relationship between new formula and the formula from the standards DL/T6201997 (Overvoltage Protection and Insulation Coordination for AC Electrical Installations) is given, aiming to identify the causes of electronic and electrical equipment damages quantitatively to provide reference for lightning protection design.