Seaplanes are special planes that can take off, land, and berth on water surface, which have been widely applied to civil and military fields. By methods of CFD calculation and wind tunnel testing, we study changes in aerodynamic characteristics caused by modifying a land-based airplane of Y12E to a seaplane, and constitute aerodynamic compensation scheme to fulfill relevant specification requirements. Our work has a certain reference value for modification of land-based airplanes to seaplanes.