The utility model relates to a length measuring device, and especially relates to a rope outgoing length measuring device of a winch steel wire rope. The rope outgoing length measuring device of the winch steel wire rope comprises a winch, a steel wire rope, a fixed pulley and a coder. The steel wire rope is extended out of the winch and is veered by the fixed pulley, and the coder is arranged on the fixed pulley and is used to measure the angular velocity of the fixed pulley, wherein the coder is an incremental encoder, the fixed pulley is equipped with a gearbox which is rotated along with the fixed pulley, and the coder is arranged on the gearbox. According to the utility model, the coder is used to measure the angular velocity of the fixed pulley, and then a linear velocity is obtained according to the relationship of the angular velocity of the fixed pulley and the linear velocity, and finally the rope outgoing length of the winch steel wire rope is obtained, so that the rope outgoing length of the steel wire rope can be controlled accurately by adjusting the load of the steel wire rope and the accepting and releasing applied forces to keep a pipe joint and an installation vessel stable. The rope outgoing length measuring device of the winch steel wire rope is simple in structure and is easy to operate.