The influence of low air density in the plateau environment on dynamic stability of projectiles was investigated. The force equations were presented in ballistic trajectory frame and the moment equations were presented in the non-rolling body frame, and the state space model of projectile angular motion was obtained by linearization method. Four eigenvalues of the angular motion state matrix were listed, and the equations of real parts of eigenvalues were derived by complex square root calculation method. The novel definition of stability factor for projectile dynamic stability was proposed, and it is proved that the new dynamic stability criteria is consistent with the traditional dynamic stability criteria. The influence of low air density on dynamic stability of fin-stabilized projectile and spin-stabilized projectile were discussed. Simulation results demonstrate that the projectile dynamic stability is different between plateau condition and plain condition.