Superplastic bulging is important technology for forming thin walled workpieces with shell shape. Especially, the compound technology of superplastic bulging and diffusion bonding, by which scores of parts or even more than one hundred parts could be combined to become a assembly, would reduce the weight of the assembly, raise its rigidity and make its structure more reasonable. So, superplastic bulging forming technology plays an important role in the field of aviation manufacturing application. Superplastic free bulging test provides important foundation for establishing mechanical theory of superplastic bulging and it also is a necessary process for superplastic bulging forming due to no die and no effect of friction. Aimed at the problems  coming from precisely measuring the geometry parameters of the axisymmetric surface of revolution which is the real shape of free bulging during the experiment, a vision measure way and its principle are brought forward to detect the edge of sample in the paper. The components of monocular vision system and the method of the camera calibration are introduced. The error compensation equation was deduced through analysis on the vision measure process of the revolution surface. In order to ensure the system in real--time, the algorithm of digital image processing adopted during the experiment is different with that after the experiment, which used sub--pixel arithmetic to improve the measure precision. And then the causes of the error are investigated.