As the most important components of marine power installation, the quality of the longaxis heavy forgings has a direct influence on the hydrodynamic characteristics and normal voyage of marine. The squaring process of the marine intermediate shaft forged by flat anvil, FM method and the unequal flat anvil was simulated. The finite element model was established based on numerical simulation software Deform3D, and the temperature field, stress distribution, forging force and forging efficiency were analyzed. The results show that comparing with the FM method and unequal flat anvil forging, when forging by flat anvil, the forming efficiency is the highest, the forging force is the smallest and the deceasing speed of the surface temperature of forging billet is the slowest; when forging by FM method, the ideal compressive stress distribution state can be obtained along the billet cross section.