Abstract: In order to obtain the center of gravity which not easy to be obtained by the theory or the experiment to certain complex rigid body, according to the rotation inertia parallel axis theorem, the relational expressions asking the center of gravity were deduced by the known axis rotation inertia and certain geometry relations, and theoretically two possibility centers of mass were got, which May be Selected after the actual judgment. So long as surveys the rigid body first with the instrument to three parallel axis rotation inertias, may obtain the center of gravity position by that relational expressions. Sometimes this method is simple compared to the direct survey the center of gravity.