Combining the principle of the equal thickness interference of air wedge with the CCD image-processing technique, a high precision automatic measurement method of thermal expansion coefficient is presented. The principle and process to trace and measure the interference fringe displacement which is caused by the position change of the either flat glass of the air wedge due to the expansion of the heated object by means of an interference fringe real-time sampling and processing system is described. With the least square method, the thermal expansion coefficient can be calculated by the relation between it and the numbers of CAD pixels passed by the moving interference fringes. The experiment result proves the feasibility of the new method. The relative uncertainty of the thermal expansion coefficient is 1.1%. Compar with the ordinary methods,the new method is more reasonable and can reduce some system error introduced by improper approximate formula.