The invention relates to the technical field of lightning arresters, in particular to a method for selecting a line lightning arrester series gap in a DC power transmission system. The method comprises the following steps: firstly, determining high-voltage side and low-voltage side electrode structures of a lightning arrester series gap according to the voltage level of the DC power transmission system; secondly, determining a DC wet withstand voltage of the lightning arrester according to the voltage level of the DC power transmission system, and proposing a minimum series gap distance underdifferent altitude conditions; finally, according to the typical tower type of the DC transmission line, determining a tower head air gap of the tower and lightning impulse discharge voltage of the lightning arrester, determining DC 1mA reference voltage of the lightning arrester according to the highest operating voltage of the DC power transmission system, proposing the maximum series gap distance of the lightning arrester under different altitude conditions, and finally completing series gap selection. Through the method provided by the invention, the accurate selection of the series gap ofthe line lightning arrester for the DC power transmission system is realized, the blindness during primary selection is avoided, and the design, manufacturing and test efficiency of the lightning arrester is greatly improved.