Abstract: With the help of Pontryagin′s maximal principle(PMP),the fuel optimal lunar soft landing problem was converted into a two point boundary value problem(TPBVP) of variable final time.Initial adjoint guess technique was introduced to provide approximate initial adjoint variables by solving linear algebra equations in the neighborhood of the initial time.Linear perturbation method,which can deal with TPBVP with variable final time,was used to solve the TPBVP.Simulation results indicate that the guessed initial adjoint variables are just in the convergence regions of linear perturbation method,and the convergence rate is rapid.The optimal soft landing trajectories varied with thrust magnitude were investigated.The results show that high-thrust can not only reduce landing time,but also brake fuel consumption.