The vacuum hot bulge forming has been used in aerospace industry to manufacture cylindrical workpiece with improved mechanical properties and reduced fabrication cost. Vacuum hot bulge forming is based on the material soften and the stress relaxation theory. Different from other metal forming techniques, deformation of the workpiece takes place well below yield point and the amount of plastic deformation is directly relaxed to heating temperature and holding time. In this paper, a two-dimension thermo-mechanical coupled finite element model was developed. In this model, nonlinear radiation heat transfer and thermal physical properties of material depending on temperature were considered. This paper carried out numerical simulation of vacuum hot bulge forming of BT20 Ti-alloy cylindrical workpiece by using finite element software MSC.Marc. The temperature field, deformation field and stress field of hot bulge forming of BT20 Ti-alloy cylindrical workpiece were calculated. Numerical simulation results were accorded with experimental ones, which provided for the practice production as theory bases.