Abstract: This paper presents an energy principle,zero different principle of coupled systems in photoelasticity,from which potential energy,complementary energy,generalized potential energy and generalized complementary energy variational principles of the coupled systems in photoelasticity are derived. What is called the coupled systems means that two deformational bodies,forwhich figures,sizes,loads and boundary conditions are the same and they are all inactual states but they are made of different materials.Prototype body and model body in photoelasticity are essentially the coupled systems,therefore the above principles become the theoretical basis of defining the influnce of Poissons ratio v on accuracy of the frozen-stress method.