The invention relates to a geometrical orbit and gesture determining method and system for deep space landing and a deep space lander. The method comprises the following steps: firstly, determining adirection vector of the lander relative to the Mars and the distance from the lander to the center of the Mars; solving the height from the lander to the surface of the Mars according to the distancefrom the lander to the center of the Mars and the radius of the Mars; establishing an inertial coordinate system xIyIzI to obtain a gesture matrix from the inertial coordinate system to a body coordinate system; further solving the inertial position and the inertial speed of the lander. According to the method and system, geometrical orbit and gesture determination is carried out by using a distance and speed measuring sensor automatically. Under the circumstance that IMU in a power falling section fails, orbit and gesture determination can be carried out by means of the distance and speed measuring sensor so as to achieve initial standard capturing of autonomous navigation, the dependent degree of the inertial navigation system to the initial standard is alleviated, and the robustness andthe failure tolerant capability of the autonomous navigation system of soft landing are improved. The algorithm is an analytical algorithm which is convenient for on-star algorithm, thereby laying afoundation for implementing a soft landing task successfully.