Smart molecular actuators have become a cutting-edge theme due to their ability to convert chemical energy into mechanical energy under external stimulations. However, realizing actuation at the molecular level and elucidating the mechanisms for actuating still remain challenging. Herein, we design and fabricate a novel nanoscaled polyoxometalate-based humidity-responsive molecular actuator {Bi8Mo48} through the assembly of [Mo2O2S2]2+ units, transition metals, and flexible phosphonic acid ligands. {Bi8Mo48} exhibits a semi-flexible cage-like architecture with oxygen-rich surfaces and highly negative charges 72-. The nanoscaled molecular actuator shows reversible expansion and contraction behavior under humidity variations due to lattice expansion and contraction induced by hydrogen bonding and solvation interactions between {Bi8Mo48} and water molecules. Molecular dynamics simulation was further employed to study these processes, which provides a fundamental understanding for the mechanism of humidity actuation at the molecular level.