In this paper, a two-step model reduction method is proposed using a strap-on launch vehicle as research object. In this method, the double-compatible free-interface modal synthesis method is first used for the modeling of the system, and the first step of reduction work is done by preserving the lower-order modes of flexible components of the system in the modeling process for the system. Then the second step of reduction is done for the established dynamic model of the system using the modal cost analysis method. After the two steps of order reduction, a low-order dynamic model of the system can be obtained. Simulation results indicate that this low-order model can reflect the characteristics of the original system effectively and the order is low enough as well.