The invention discloses a controllable metal oxide lightning arrester for a power transmission system, belongs to the technical field of electric power engineering equipment, and solves the problems that in the extra-high-voltage power transmission system, an existing metal oxide lightning arrester is poor in heat dissipation effect and easy to explode. According to the technical key points, a lightning arrester main body is connected in parallel with an external thyristor switch; the thyristor switch is in signal connection with an external signal receiving end; resistor sheet groups are arranged from top to bottom in a form that the upper part and the lower part are dense and the middle part is sparse; the resistor sheet groups are surrounded by a heat dissipation layer; a cavity is formed between the heat dissipation layer and the resistor sheet groups; an expansion ring protruding outwards is arranged on the side wall of the middle of the cavity; heat dissipation holes are uniformly distributed in the expansion ring; a heat conduction cavity is formed in the inner side of the expansion ring; and insulation jackets are sequentially arranged on the outer side wall of the heat dissipation layer from top to bottom in the vertical direction at intervals. According to the lightning arrester disclosed by the invention, the heat dissipation speed of the lightning arrester under a high-temperature condition is increased, so that the internal gas pressure and temperature of the lightning arrester are prevented from being increased sharply, and explosion accidents are reduced.