Abstract: Absolute test eliminates the surface errors of reference plane of interferometer, so that the inhomogeneity of optical materials can be measured with high accuracy. A brief review and comparison of several mainstream absolute test methods for inhomogeneity of optical material were given. And influence of the factors, such as the pixel offset, resolution of interference pattern, repetition of interferometer, the sample thickness and the refractive index measurement, etc, on the measurement error of absolute test was analyzed respectively. Experimental results indicate that the measuring accuracy of window-flipping method, transmission method and parallel flat method can reach 10-8 respectively. Repetition of interferometer is the main error in absolute test method of the inhomogeneity of optical materials.