Abstract: A time-dependent analysis, coupling the unsteady N-S equation, beam vibration equation and angular motion equation, was formulated to numerically simulation flight motion of an elastic large fineness ratio projectile. This method was used to study the influences of stiffness and structure damping on the flight of projectile. The result indicated that the unsteady aerodynamic induced by vibration was too small to change angular motion of the projectile. A divergence in flight motion was observed when steady state rotation speed of the projectile, with slight radial center of gravity offset, is near angular motion frequency.