The early warning of satellite is a necessary process and an important part of the early warning system, which is divided into double satellite detection and single satellite detection. The traditional single satellite detection method is based on the prior information of standard trajectory template, but the standard trajectory template information is difficult to obtain.In this paper, a missile trajectory estimation model by early warning satellite based on direction constraint is proposed aiming at the problem of trajectory estimation of ballistic missile by early warning satellite. The trajectory of ballistic missile is estimated and predicted by using the detection data of the missile boost phase of single satellite and double satellite detection respectively , and the quantitative calculation is carried out. During single satellite detection, the smaller the angle between the satellite position and the missile plane, the greater the error. When the angle reaches more than 5 °, the error is small, which can basically meet the position guidance requirements of long-range early warning radar. Double double star detection, The prediction error is further reduced and the accuracy is further increased during.The above simulation results show that the trajectory estimation accuracy of this method can provide enough target guidance accuracy for ground-based early warning radar.