Glass bending is an important process in glass device manufacturing, and real-time stress detection is essential for regulating bending mold pressure. A three-point bending experiment is used to simulate the glass cold bending forming process, and the feasibility of the photoelastic method to detect the real-time bending stress in the pure cold bending state is analyzed. The conventional division-of-amplitude polarimeter is optimized to obtain a smaller spot diameter, fewer system conditions, and a wider stress detection range. The Stokes parameter error of the optimized system is 0.05, and the optical path difference measurement accuracy is 4.284 nm. The test quality of the photoelastic stress detection system during the three-point bending was not affected by the glass size or bending span. However, the various curvatures of the incident surface were found to cause different stress values in unloaded glass and fluctuations in y-values (the distance from the test beam to the neutral surface of the glass) to occur outside the normal range. The less the curvature of the glass is, the closer it is to normal incidence, and the more accurate the stress value is. After the heat treatment of the borosilicate glass, the experimental results show that the isotropic structure of the glass does not change significantly. The exploration of testing quality of bending stress provides an effective reference for practical high-precision bending glass manufacturing.