In view of the problem that many errors of residual flight time estimation algorithm in terminal-guidance resulting in low guidance precision, the ballistic trajectory method and the missile-target sight average speed method were proposed to estimate residual flight time accurately. Speed ballistic trajectory method is to calculate the residual flight time through the ballistic trajectory segmentation integral. Missile-target sight average speed method is to calculate average speed on the missile-target sight on line to solve the residual flight time. The simulation results show that the new algorithm has high estimation precision by applying it to the optimal aircraft terminal-guidance law under constraint conditions.