We propose a laser frequency splitting method to determine the relative stress-optic coefficient and internal stresses in Nd:YAG crystals with high resolution. In this method a mirrored Nd:YAG crystal in a circular disk shape is made into an all-internal cavity laser. Once diametral compression force is applied, due to intracavity photoelastic effect, the single laser frequency is split in two. Through measuring the beat frequency of the split frequencies, the relative stress-optic coefficient and internal stresses in the Nd:YAG crystal can be determined according to the proportional relationship. In our experiment the measured value of the stress-optic coefficient, $C_{111}=1.27\times {10}^{-12}{\mathrm{m}}^2/\mathrm{N}$ , is close to the theoretical value; the principal stress difference at the center of the Nd:YAG crystal was also determined. The optical path retardation resolution is approximately $1.5\times {10}^{-8}\lambda$ , and correspondingly the principal stress difference resolution is approximately $5\text{\hspace{0.17em} Pa}$ . The resolution is approximately 5–8 orders of magnitude higher than those of conventional methods, therefore even very small residual stresses can be easily determined with this method. Although this method is a kind of point-by-point method instead of a full-field method, it is a promising novel photomechanics method with attractive advantages such as extremely high resolution.