The invention discloses a lightning arrester of an insulator for a power supply line, in particular relates to an outer-gap lightning arrester. The outer-gap lightning arrester is characterized in that an upper metal tool is provided with a pressure adjustment element, a conductive band, a zinc oxide resistance card, an inner discharge electrode, an insulating gasket and an outer discharge gap rod, wherein a thread upper part of an lower end metal tool is provided with an outer discharge gap sheath, and the shape of the outer discharge gap rod is drooping. The outer-gap lightning arrester disclosed by the invention has the advantages: a tiny residual voltage is introduced into the earth and the protection process of electric equipment is finished when the outer discharge gap rod generated by lightning discharges to the outer discharge gap sheath; therefore, the investment cost and the maintenance cost are reduced; the outer-gap lightning arrester is not influenced by the climatic environment, the atmospheric pollution and the geological conditions; the zinc oxide resistance card is not corroded, and the service life is long; the product made of a silicon rubber material has the advantages of lightness, portability, good insulation property, long creepage distance, convenience for construction, reduced labor intensity and safety, stability and reliability for operation. The lightning protection level of the power supply line is improved, and good effects of small investment, large yield, free-maintenance and over 98% of effective protection probability are achieved.