During the take-off of the seaplane on the water surface, one of the most dangerous situations is the uncontrollable porpoising of the seaplane. The coupling effect of the buoyancy, hydrodynamic resistance, hydrodynamic lift, and aerodynamic force of the seaplane is studied to better meet the requirements for porpoising in airworthiness clauses. In this study, we established the dynamics model of seaplane motions on the water surface and proposed a method for the simulation evaluation of seaplane porpoising. The program for simulating seaplane planning on the water surface was developed. Through simulation, we evaluated the possibility of porpoising when a seaplane plane is on the water surface and obtained the pitch attitude boundary of seaplane porpoising. The evaluation method can be adopted to develop procedures for seaplane take-off control and for risk identification and control in seaplane take-off test flights on the water surface.