Abstract: Simplification of the mathematical model for a marine gas turbine propulsion system is studied by modal analysis,and selection of remained state variables in the order-reduced modelis considered. The dynamical characteristics of the system are evaluated according to the relationship between the state and the modal variables. The ratio of the output variation to input variation is obtained from the sensitivity snalysis, and then the state vaviables to be remained are determined. The modal variables are adopted to simplify the model,finally they are reduced to remained state variables. Numerical evulation show that this order-reduced model is much better than the simplified model obtained directly from the state equations.