This paper studies the attitude stability of amphibious aircraft on water under severe winds and waves based on a nonlinear model of rotor coupling and vector projection error. Constructed a coupled dynamic model of amphibious aircraft ship mode including rotor power, and defined a control objective about pointing stability based on the vertical vector angle between coordinate system. A nonlinear PD controller about pointing stability was designed through Lyapunov stability theory, where an energy function combining the potential term of error vector norm and the kinetic term of angular velocity error is constructed. The asymptotic stability at equilibrium is rigorously proven by analyzing the derivative properties of the Lyapunov function. Simultaneously an inner and outer loop sliding mode controller (SMC) for attitude angle decoupling was designed based on control error. Numerical simulations demonstrate the effectiveness of two control methods in resisting severe wind and wave interference. While SMC exhibits better dynamic performance in disturbance suppression.