A technique for automatic measurement of the refractive index of liquid was proposed. The approach based on the principle of the equal thickness interference of air wedge and charge coupled device (CCD) image-processing technique. The unitary linearity regress method was used to fit the relation between the serial number of the CCD pixels and the serial number of the intensity maximum of interference fringes, and the regress coefficients were used to compute the refractive index of liquid. Some factors affecting the quality of fringe patterns were taken into account and perfect fringe patterns were obtained by removing background light. The experiment result proves the feasibility of the new method. The relative error of the refractive index of water is 0.09%. In order to obtain clear interference fringes in other optical experiment, the new method of removing background light is worthy to be considered.