The legged lander is the key equipment of deep space soft landing detection whose current version does not have the mobility capability making its one launching mission’s patrol range limited. In this work, a legged mobile lander is designed and studied. The complete kinematic model is established for a single leg of the legged mobile lander firstly. Then, the working mode of the legged mobile lander is introduced. To make the lander adapt to the complex terrain on the surface of planets, both periodic and free gait planning methods are proposed. The other problems like stability criteria, terrain evaluation as foothold, and foot trajectory planning are also investigated with consideration of the characteristics of the legged mobile lander. Finally, numerical simulation experiments demonstrate the feasibility and stability of the proposed motion planning method.