Abstract: Aimed at the multiple-constraint trajectory optimization problem in the descent phase of small body landing exploration, Gauss pseudospectral method is used for the optimization design and the fuel-optimal descent trajectory is derived. The optimal control model of small body descent trajectory optimization problem is established and discretized using Gauss pseudospectral method. The problem is then solved by transforming into a nonlinear programming problem. Mathematical simulation results show that all the constraints are satisfied, the optimization index of fuel consumption is minimized and the spacecraft reaches the target landing site with zero velocity. Using Gauss Pseudospectral method, the computation process of small body descent trajectory optimization is fast and has high solution accuracy.