Couplings in helicopter flying with a slung load are analyzed via aerodynamic derivatives and natural motional modes. Firstly, a fully coupled flight dynamics model including a helicopter, an inelastic cable, and a rigid body slung load is established. Then, the trim properties and frequency response properties are calculated, and they are compared with flight test data for model validation. Finally, the mutual coupling between the helicopter and the slung load system is analyzed with an example. It shows that, the coupling is mainly caused by the motion of helicopter attachment point and the force acting on it. Because of the coupling, there is a stabilization effect of helicopter attitudes on its angular motion, the damping of helicopter itself has effects on both the helicopter & the slung load system simultaneously, and cable angles have a stronger stabilization ef fect on pendulum motion. As the aerodynamic derivatives change, the helicopter roll and Dutch roll modes are destabilized while two pendulum modes are stabilized. And the lateral-directional coupling is stronger than the longitudinal coupling.