To build the launching dynamic model of the inclined launching rocket projectile which has large slenderness ratio, in addition to considering the impact of external factors, the effect of transverse vibration on initial disturbance caused by the elastic deformation of the rocket projectile body needed to be considered as well. To study the effect of crosswise vibration characteristic on motion of pitch angle in launching progress, a rocket projectile's transverse vibration model was built using transfer matrix method and multi-rigid-body dynamics simulation and multi-rigid-flexible-body dynamics simulation were conducted. By comparing the simulation results with the test results, it was found that the projectile body's structural deformation had apparent effect on pitch angle motion in launching progress.