This paper makes an analysis of the influence of the three-dimensional effect upon the measuring accuracy of the two-dimensional photoelastic techniques. On the basis of Aben's theory of the propagation of polaized light in the photoelastic medium, a new concept "quasi-Plane character of the plane photoelasticity" and a new "Quasi-plane"measuring method in the plane photoelasticity are derived for the first time. The new method eliminates the measuring error introduced by the three-dimensional effect. Hence high-precision is assured. Examples of experiment demonstrate the reliability and practicability of the new method.