Calculation of massbalancing for civil aircraft’s transonic flutter models should consider the unparallel ribs and rib’s nonperpendicular to the elastic axis. Now, an optimization linear splitting method of mass and inertia properties based on the direction of elastic axis is put forward. And that method is extendedly applied to the calculation of flutter models’ spanwise location of the centre of gravity. Some civil airplane’s vertical tail transonic flutter model used the method in massbalancing. Test results show that the related vertical tail transonic flutter model simulates the mass and inertia properties precisely, and the model’s dynamical properties meet the design objectives well. Practice indicates that the optimization method fulfills the technical standards of civil airplane’s transonic flutter model design and related airworthiness regulations, and it can be extensively applied to the calculation of other transonic flutter models’ massbalancing.