The invention relates to a built-in gravity-type safety valve for a mobile-type liquid ammonia and liquid chlorine tank and a using method. Two gravity-type safety valves are fixed to the interiors oftwo holes in a sealing head in the liquid ammonia and liquid chlorine tank and correspond to valve base holes, and control valves are in screw joint with the two valve base holes respectively; a steel gas-phase pipe and a steel liquid-phase pipe are fixed to the interiors of medium channel lengthening butt-joint ports of the two gravity-type safety valves respectively, and the free ends of the steel gas-phase pipe and the steel liquid-phase pipe conduct spot-welding positioning with the inner wall of the liquid ammonia and liquid chlorine tank separately. Under an abnormal situation or in anaccident state, the liquid ammonia and liquid chlorine tank which is vertically placed is passively or actively toppled and overturned or overturned, so that extension bodies of the two gravity-type safety valves are in a state of tilting upwards, spherical valve elements are in contact fit with conical sealing faces to form a linear seal fit pair, small-diameter medium channels are closed, and thus the automatic blocking function for leakage is achieved.