The prototype of spatial trussed lunar lander was taken as a researching object. It was designed and manufactured by RCAMC HIT(Research Center of Aerospace Mechanism and Control, Harbin Institute of Technology). The simplified model of the lunar lander construction was given. Then, the location coordinate equations of every lunar lander foot and center of mass in inertial coordinate system were deduced. Based on these location coordinate equations, the lunar-s quasi-static stability condition was gotten. By analyzing the lunar lander-s transient dynamics behavior during the landing time, the formulas to calculate the transient dynamic parameters of the lunar lander during the landing time were derived; Through scattering time variables, the calculating procedures that can be imitated dynamic characters of the whole landing process were put forward, which laid a foundation for further research on the lunar lander-s landing dynamics behavior.