Circular bubble experiments were carried out to study the biaxial tensile mechanical property of ETFE film. A full stress strain curve was investigated and basic mechanical parameters of ETFE film were obtained. The results show that true stress strain curve reaches first turning point when true stress is about 1718MPa which is same as that obtained by uniaxial tensile test, the curve tends to leveling off when true stress is about 50MPa, and then reaches the maximum value when true stress is about 60MPa because of breaking of bubbles. The true strain obtained by bubble test is 30%40%, which is much less than that of uniaxial tensile test. Based on the test results, a four polygonal line model is put forward and its applicability is verified by numerical simulation analysis.