The invention discloses a device for enabling a tail fiber to pass through a tube and an application method for the device, and relates to the technical field of tail fiber tube-threading. The invention aims at solving problems that the tube-threading operation of a tail fiber is difficult, wastes a lot of time, and affects the work efficiency. According to the device, a tail fiber connecting part is connected with the tail fiber, and a lead weight is shaped like a water drop. The maximum external diameter of the water drop is less than or equal to 15mm. The tail fiber connecting part is fixedly connected with one end of a nylon line, and the other end of the nylon line is fixedly connected with the tip of the lead weight. The device is suitable for optical signal transmission equipment to carry out threading of the tail fiber through a tube, such as the threading of tail fibers, employed by an optical transmitter and receiver, an information exchanger, a relay protection device, and an automation device, through the tubes between the tail fibers and an optical distribution frame.