Aiming at the problem that the online trajectory planning of high-speed unmanned aerial vehicle (UAV) in the ascending phase needs to realize online fast solution under multiple constraints, firstly, the motion and dynamics model of the vehicle was built, and the constraints faced by the trajectory planning were given. According to the constraints and flight characteristics, the action state space and reward evaluation function that meet the mission requirements were designed based on the near end strategy optimization (PPO) strategy gradient optimization. Secondly, based on the characteristics of strong time memory of the trajectory planning in the ascending phase of the aircraft, the short and long term memory network (LSTM) network structure was introduced on the basis of the traditional PPO algorithm, and the PPO-LSTM algorithm was used to solve the online trajectory planning problem in the ascending phase of the high-speed aircraft, and the model that can plan the optimal angle of attack strategy in real time according to the aircraft state was trained. Finally, the performance of the algorithm was verified by Monte Carlo simulation. The results show that the root-mean-square error of the terminal state of the algorithm in this paper is reduced by about 50% compared with the traditional PPO and particle swarm optimization, which fully proves the superiority and effectiveness of the proposed algorithm.