In this paper a new method of measuring some parameters of photoelasticity is presented, and the heterodyne interferometry has successfully been applied to the field of photoelasticity of selected optical materials. A device which brings atmosphere to bear on the optical materials has been developed to measure the change of refractive index at different pressures, and the relationship between the given pressure and the corresponding change of refractive index has been obtained through a series of experiments. Also, a careful analysis is given to the results of the experiments. The simple device and the easy method can make the last measuring precision of change of refractive index reach 8.4×10 -8 with small stress applied.