A nonlinear dynamics model is built for analyzing dynamic stability of coaxial helicopter. In the case of ignoring the nonlinear aspects of lag damper and landing gear, the range of rotation speed of ground resonance is confirmed by calculating the time domain response of blade lag and airframe motion with different rotation speeds. Then it is compared with the result calculated by eigenvalue method. Finally, the time domain responses of blade lag and airframe motion with different rotation speeds are respectively calculated by the nonlinear dynamics model and the linearization model. The results show that the nonlinear aspects have little effect on blade lag and airframe motion in stable area, but in unstable area, the response has a significant difference between the nonlinear system and the linear system, and the nonlinear system appears limit cycle phenomenon.