Abstract: Time-optimal and fuel-optimal soft-landing trajectories of lunar probes with constant-amplitude thrust are investigated. System model of soft-landing trajectories is established, and parameterization technique and Sequential Quadratic Programming (SQP) method are applied to the normalized model to obtain time-minimum and fuel-minimum trajectories. An integrated study is carried out between two stages of lunar soft-landing trajectories - Keplerian trajectory and continued-propulsion descent trajectory. When lunar probes descend from 100km height to 15km height, the Keplerian trajectory has around 15km high pericynthion. Simulation result shows that there is a point near the pericynthion, and if the continued-propulsion descent trajectory begins at this point other than the pericynthion more fuel will be saved.