In this paper, optimal trajectories of soft landing on the Moon are designed based on different landing strategies. For this purpose, the problem of soft landing is defined as an optimal control problem to minimize fuel consumption and solved by a combinational direct method. The used solution method in this paper is a combination of direct collocation method, nonlinear programming, differential flatness and B-spline curves. In this method, by using differential flatness, dynamic equations of landing are expressed by the minimum number of state variables in the minimum dimensional space. Also, state variables are approximated by B-spline curves, and control points of these curves are considered as optimization variables of the nonlinear programming problem. By simultaneously using of differential flatness and B-spline curves, the number of variables and constraints of the optimal control problem decrease significantly and the problem is solved with high accuracy and speed. In the paper, three different strategies for soft landing on the Moon are investigated. These strategies are defined based on direct or indirect landing from the parking orbit and separation of horizontal braking and vertical descent phases. According to achieved optimal trajectories, by indirect landing from an intermediate orbit, the space vehicle can be landed on the Moon with the minimum fuel consumption. Also, by separation of horizontal braking and vertical descent phases, a more applicable landing trajectory can be achieved.