The present invention discloses a device and a method for detecting the tension on a guide rope of a hanging scaffold in a construction shaft. The guide rope is released by a winch, rounds over a hoisting sheave, is connected to the hanging scaffold, and then is tensioned up; the hoisting sheave is disposed at a position above the winch, and the device comprises a slide device, two tension ropes, and a pull rope, wherein, the slide device is fitted around the guide rope, the two tension ropes are fixed to the two sides of the slide device respectively and arranged parallel to the guide rope, the pull rope is fixed to the lower part of the slide device and arranged perpendicular to the guide rope. On the basis of the basic principles of mechanics, the force applied on the guide rope can be calculated indirectly according to the proportional relation between the forces applied on the tension rope and guide rope and the lengths of the ropes. The device disclosed in the present invention is simple, and the method disclosed in the present invention is skillful. The device and method are applicable to thick and thin steel wire ropes and highly universal, and are low in cost.