The authors present the principals, the measuring devices and method of a biaxial photoelastic strain gauge. It has been pointed out that the optimum position of measurement in the strain gauge depends upon the Poisson's ratio of the gauge material, only. Within a given positioning tolerance of ±0.25mm for the optimum measuring point, the maximum error of the measured differential principal stress is ±4%. Experiments show fringe patterns in the strain gauge with two clear and precisely perpendicular axes of symmetry, which indicate the directions of the principal strain. The biaxial photoelastic strain gauge made of proper materials with an outer diameter of 50mm, an inner diameter of 10mm, and a thickness of 4mm has a sensitivity of 6.45με. It provides a means of convenient, economic, and fast in-situ stress measurement.