The Chinese nickel-base powder metallurgy (PM) superalloy FGH96, which was processed through hot isostatic pressing, is very difficult to deform. FGH96 superalloy has better superplasticity in special deformation conditions and superplastic isothermal forging is the best formation method at present. The accurate constitutive equations of the FGH96 alloy was established depended on the isothermal compression experiments. A two dimensional and thermomechanical coupled axisymmetric finite element(FE) model in which both part and die were taken in consideration was established to fully simulate the FGH96 superalloy turbine disk superplastic isothermal forging process. Some physical parameters about the turbine disk forging process, such as load, stress field and strain field were calculated at different temperature within the forging range of FGH96. The regularity of peak equivalent stress acted on die cavity surface, yield limit and ultimate strength of die material during the forging process was found. Based on the regulation, peak equivalent stress acted on cavity surface must be extremely less than yield limit of die material, the optimized processing parameter 1050°C that is the best deformation temperature for the alloy was determined. That was proved better in practice and high quality disk was forged.