FMS is the core of the modern civil avionics system.With PBN to be spread around the world,the trajectory prediction function becomes an essential part of FMS.The vertical prediction dominates the prediction procedure which calculates the states of the aircraft by using performance models.This article analyses how the core functions and databases of FMS cooperate with each other to compute the predicted trajectory,and shows the relationship between the vertical prediction and the lateral one.With brief introduction of performance models used in the computation,the typical vertical profile and vertical prediction procedures are elaborated in the following chapter.At last,the accuracy of the algorithm is proved with the flight simulation from Lhasa to Nyingchi.