Based on the multi-body method, a rotor/ fuselage /landing gears coupled nonlinear aeroelastic dynamic model during rotor starting process on ground is established, in which both the variations of inertial and aerodynamic forces due to the increase of rotor rotation speed and the elongations of landing gears due to the rise of gravity center of helicopter are taken into account. Besides the transient responses of fuselage and blades under different pitch control in normal starting conditions, the accidental responses are calculated with an example for the break of a lag hinge during rotor starting process. Numerical results show that the established model has ability to simulate the transient responses of helicopter during rotor starting process.