We have studied the properties of the classical dynamics of three ions with collinear configuration in a Paul trap.We found that no chaotic motion exists,and the motions of the ions are regular at any given energy E.The trajectories of the ion ensemble in phase space are of the anti-symmetric type and symmetric type periodic(or quasi-periodic) orbits,which are constrained in two KAM tori respectively.The simplest trajectories of the ensemble are periodic orbit A and periodic orbit S.When E tends toward the minimum energy Emin=3.0 of the ensemble,the motions of A orbits and S orbits tend towards the anti-symmetric and symmetric harmonic oscillations,respectively.