The invention belongs to the technical field of submarine line laying equipment, in particular to a track assembly positioning mechanism of a four-track tensioner system. The track assembly positioning mechanism comprises a limiting sensor (1), a pulley assembly (25) and a positioning base (4), wherein the pulley assembly (25) comprises an axle seat (2); a core shaft is fixedly mounted on the axle seat (2), and is sleeved with a shaft sleeve (3); the axle seat (2) is fixedly mounted at one side, close to a right track frame (28), of an upper track frame (24); the positioning base (4) is fixedly mounted at the top of the inner end of the right track frame (28); a V-shaped groove is formed in the top of the positioning base (4); the inner surface of the V-shaped groove extends towards the inclined upside of the inner side to form an inclined guide face; and when the shaft sleeve (3) slides to the bottom of the V-shaped groove along the inclined guide face, the limiting sensor (1) senses the position of the axle seat (2) and produces a trigger signal. The invention also relates to a positioning method of the track assembly positioning mechanism.