Although flying boats have been studied for over one hundred yeas, simulation of flying boats in waves and controllers designing for flying boats are still new research fields. After analyzing the forces acting on a flying boat, the flying boat is modeled in this paper as a special airplane. The forces from water are discussed and calculated by the 2-dimensional (2D) strip theory which is widely used in the force approximation of planing crafts. And the model is validated by experiment data, which shows the mathematical model has an acceptable performance on the motion-prediction of the flying boat in water with regular waves.