Abstract: A method for precisely detecting a width of a crack on a surface of an object is provided. The method includes positioning a crack width observation apparatus on the object, turning on a first light source at a first side of the crack, and photographing the surface with the first light source on to generate a first photograph to determine a boundary of the crack at the side distanced closest to the light source. A second light source at a second side of the crack is turned on to irradiate the surface of the object, and the surface is photographed with the second light source on to generate a second photograph to determine a boundary of the crack at the second side. The first photograph and the second photograph are processed to obtain a picture showing an actual boundary of the crack.