Abstract: Residual gas noise limits the measurement accuracy of space-borne inertial sensors and is critical for space-borne gravitational wave detection. The dominant residual gases are hydrogen and water molecules in inertial sensors during on-orbit operation. The evaluation of water-molecule-induced noise remains a major challenge. Water molecules exhibit long and widely distributed surface residence times on metal surfaces, ranging from 10⁻³ s to 10⁵ s. This behavior prolongs the impulse intervals acting on the test mass and interferes with gravitational wave signals in the 0.1 mHz to 100 mHz range. This phenomenon is known as the dynamic adsorption–desorption effect. In this study, a theoretical model of residual gas noise induced by the dynamic adsorption–desorption of water molecules is developed, and its physical mechanism is analyzed. The results show that the residual gas noise induced by water molecules is 1.73 times that induced by hydrogen, and it may exceed the threshold of 1×10⁻¹⁵ (m·s⁻²/Hz^1/2). The dynamic adsorption–desorption effect is governed by the adsorption coefficient k_a and the desorption coefficient k_d. Adsorption coefficient k_a reduces noise by buffering momentum transfer, while a higher desorption coefficient increases it. This study and its findings are significant for the space-borne gravitational wave detectors.