The process design of profile ring rolling for a large-scale turbine diaphragm was made using the calculation method and three dimensional finite element method (FEM). The design criteria are to achieve uniform distributions of strain and temperature, and defect-free profiled ring products. Based on the compression test results of the low alloy steel, deformation processing map was generated using the combination of the dynamic materials model (DMM) and stable or unstable forming criteria. The processing map was used to determine the optimum ring rolling temperature and feed rate of the mandrel. FEM analysis was simulated to predict the formation of rolling defects and deformed shape in the profile ring rolled diaphragm product. Finally, optimum process design to obtain a sound large-scale turbine diaphragm without forming defects was suggested and its validation was made by the comparison between the experimental data and FE analysis results.