When radar is tracking cluster ballistic missile targets which include warhead, decoys,etc. if an important radar tracking is lost, trajectory prediction is often carried out to maintain the track. The accuracy of the prediction depends on the accuracy of the initial accuracy. The relative error between multiple targets is generally less than the absolute error of a single target. To take advantage of this feature, this paper presents a prediction method to maintain the lost track, based on the relative motion relation among ballistic targets. The restrictive condition is that the cluster targets do not maneuver between each other. Through error analysis, the mechanism of improving the accuracy of trajectory prediction is studied quantitatively. Simulation results show that the accuracy of trajectory prediction is greatly improved, while the radar has good correlation with multi-target detection information.