An accurate reference trajectory of the terminal area energy management (TAEM) phase can enable a reusable launch vehicle (RLV) to land in the planning area more accurately. In this paper, an online generation method of TAEM phase longitudinal trajectory based on offline trajectory library is proposed. Firstly, it is assumed that the point state and longitudinal aerodynamic coefficients of the RLV starting descent in the actual flight process conform to the normal distribution, and the appropriate nodes are selected to constitute the list of nodes according to the principle of equal probability. Secondly, the list of nodes is simplified by judging the linear relationship of adjacent nodes of each variable, and combining the simplified nodes, the construct the offline trajectory library. Finally, the online trajectory generation process based on the offline trajectory library is given. Simulation results show that the method in this paper can effectively simplify the construction process of the trajectory library, and can effectively plan the longitudinal trajectory of TAEM phase with small landing error in real time.