A three-dimensional online trajectory planning method is proposed to solve terminal-area energy management of reusable launch vehicles. This method includes three important aspects. First, the angle of attack profile used is programmed by a two-dimensional longitudinal dynamic equation in the energy domain instead of a three-dimensional dynamic equation. This reduces the amount of calculation and improves the calculation speed, which is suitable for online application. Second, this method uses geometry to plan the ground track, which improves the applicability of the method to the ground trajectory with complex shape. Finally, the prediction-correction technique is used to compensate the range loss caused by lateral motion. These techniques make the algorithm converge quickly in three-dimensional trajectory planning and the technical indexes such as range can reach the design value. A numerical simulation verified this method's rapidity and high precision.