Optical homogeneity is one of the most important parameters of optical material. The traditional absolute testing method is considered to be the high-precision way of optical homogeneity measurement. However its corresponding operation procedure is very complex. So it is sensitive to the environmental fluctuation. Based on the characteristics of wavelength tuning, two-step wavelength tuning absolute testing method of the optical homogeneity is presented. It only consists of two measurement steps, with and without the parallel plate as the material in the test arm cavity between transmission flat and reflective flat. The presented method is verified in terms of numerical simulation. And the measurement experiments with both the traditional absolute testing method and the presented method are carried out. In the end, the experimental result shows the validity of the presented testing method. After comparison, it is clear to see that the presented method is of high precision, which is simpler than the traditional one.