Based on the principle that the Michelson interferometer can be used to measure the slight variation in length, a device for accurate measurement of thermal expansion coefficient of solid material line by using the method of equal optical path is revised and designed. The basic idea is that two pieces of mirror on original Michelson interferometer are converted into moving mirror, are put on the new added rail, and are connected with one end of the tested metal rod. When the metal rod is heated, it will lengthen slightly, which drives the moving mirror to produce a slight movement. By adjusting and observing the demarcation point of the interference fringes’ “indentation” and “emission” on the observation screen before and after heating, the micro change of the shape of the metal rod can be determined.?And then, the coefficient of linear thermal expansion of the tested metal rod is calculated according to the formula. The method overcomes the defects of traditional fringe method, and can realize the accurate measurement of the coefficient of linear thermal expansion of the metal rods. This method has the advantages of simple in operation and practical.