The purpose of this study is to develop the simplified model of the crankshaft in the large marine engine for dynamic analysis. Because the actual engine system is under complex dynamic loading condition and it has multi-cylinder, the dynamic analysis is purchased at a high computation cost. In spite of this burden, the dynamic analysis must be perfonned to assure structural integrity of operating marine engine. Therefore, simplification of the analytic model is necessary for dynamic analysis. Beam-mass model, which is generated with the section property method, is the model simplified effectively. Section property method can provide desired section information by optimization technique. By applying beam-mass model to the crankshaft in the large marine engine, the usefulness of the proposed method was proven.