Abstract: Determining how to find the best controls of a re-entering reusable launch vehicle (RLV) so that it is able to safely reach the terminal area energy management (TAEM) involving the solution of a two-point boundary value problem. This problem, which is considered to be difficult, is traditionally solved on the ground prior to flight. The optimal controls are found regardless of computation time by most of algorithms. But it-s very necessary to find the optimal controls quickly for some flight tasks. Traditional trajectory optimal algorithm can not shoulder this fast optimization task. In that work, a new hypothesis was introduced according to the features of constrained three-dimensional reentry trajectory of RLV. The set of dynamics and kinematics equations of motion was divided into two sets and only one set equations participate in the optimization iteration algorithm, which improves the efficiency of optimization greatly. Then the methods of multipliers was used to deal with the terminal constraints. Later the conjugate-gradient method was applied to evaluate the optimal reentry trajectory. Successful results show the algorithm is able to generate a feasible reentry trajectory in 10 s on the desktop computer.