Open die hot forging has a wide industrial application on deforming ingot into billet with desired dimension and qualified internal microstructure. An example open die forging process of Ti-6Al-4V ingot is selected herein. A 3D FE-based numerical method was used to investigate the open die forging process with respect to the real working conditions. The simulation results were validated by the collected experimental process parameters from the forging system. Moreover, design of experiment method is adopted regarding the variation of process parameters to reveal the effects of critical factors on product deformation and quality characteristics. Results show that the process parameters including press speed, feed and reduction has significant effect on the workpiece deformation and effective strain which represents the forged billet formability and quality. Improved process parameters method is suggested with respect to the experienced benchmark based on the sensitivity analysis. Keywords: Open die forging; Ti-6Al-4V alloy; Sensitivity analysis; Process parameter; Numerical simulation;