Abstract: It is one of the key technical problems for determining residual stress by means of photoelastic method to clarify the relationship between residual stress and birefringence fringes, i. e. isochromatic fringes caused by frozen stress and thermal stress, in polymethylmethacrylate （PMMA）.The relationship is investigated by means of annealing and making a cut in frozen stress specimen only containing the fringe caused by frozen stress and in the first slice of thermal stress specimen prepared by cooling only. It is concluded that the residual stress in PMMA depends only upon the thermal stress caused by cooling, and is irrelevant to the loads added at high elastic state and to the amount of the fringe caused by frozen stress, and furthermore the relationship between the residual stress and the amount of thermal stress fringe conforms to the stress-optical law. Equa-tion（ 3 ）is the expression for such relationship and can be used to determine the residual stress quantitatively in engineering. In illustration of application, the distributions of the residual stresses in PMMA specimens cooled at different speeds are examined （Fig. 9 ）, the residual stresses in two different types of canopies are determined （Fig. 10）, quantitative relationship between annealing temperature and residual stress is established （Fig.11）, and distributions of the residual stresses in DYB-4 PMMA in different parts of a canopy are considered. It is shown that the conclusions drawn in fhis paper may be useful for quantitative determination of residual stress in PMMA, and benefit the quality control, life prediction and failure analysis of PMMA airplane canopies.