An adaptive feedforward controller has been developed to address the impact on flight stability of variations in the moment of inertia caused by mechanical leg movements during the flight of amphibious unmanned aerial vehicles (UAVs). Firstly, a dynamic model of a hexacopter UAV was established. Based on the variable range of the motion angles of the mechanical legs, the variation interval of the moment of inertia was determined, and a set of mechanical leg motion paths covering the maximum, minimum and extreme variation values of moment of inertia was designed. This converts the relationship between the moment of inertia and the time into a nonlinear function. Furthermore, given the variable moment of inertia of amphibious UAVs, a control strategy based on feedforward control was developed. Finally, a simulation in Simulink was carried out to compare feedforward control with the traditional proportional-integral-derivative (PID) control under the conditions of a dynamically changing moment of inertia. The results verified the superior performance of our feedforward controller in dealing with variations in the moment of inertia of amphibious UAVs.