A trajectory forecasting algorithm of boost-glide unpropulsive skipping vehicle is designed based on the fixed character of the vehicle's lift-drag ratio. Firstly, by analyzing the track characteristics of this kind vehicle, the reentry-uprising time is determined as the beginning time of this algorithm because of the vehicle's unique reentry-uprising phenomena. Secondly, through integrating the known information with the unknown information to the defense system, the changed movement equation and algorithm of the unknown parameter are given. Based on the changed movement equation, the track forecasting flow and algorithm are designed. Finally, the simulation is done and the result shows that the forecasting algorithm has a better ability to forecast the latter trajectory when the vehicle is uprising again.