Abstract: To meet the need for fast and interpretable in-orbit evaluation of the self-gravity disturbances acting on the test masses (TMs) of space-based gravitational-wave detection spacecraft under propellant consumption–induced mass distribution changes, this paper investigates the coupling effects among the spacecraft center of mass (CG), self-gravity center (SG), and test mass (TM) under fuel-consumption conditions, and proposes a coupled fast-prediction approach. First, finite-element analyses are performed to quantify how the spacecraft geometry and the propellant-consumption–driven mass redistribution affects the SG, yielding a dataset that includes CG, SG, and related quantities. Second, piecewise linear coupling matrices for “CG–SG” and “CG–TM self-gravity” are identified, and singular value decomposition (SVD) is used to verify the near one-dimensional evolution of the CG, thereby reducing the problem to a scalar parameter. Finally, based on this scalar parameter, a piecewise cubic Hermite interpolating polynomial (PCHIP) mapping is constructed to enable rapid prediction of the SG position and the self-gravity increments at the TMs. Results show that, within the test intervals, the proposed model maintains good consistency even in fuel-consumption ranges not used for model construction: the PCHIP prediction error of the SG remains at the millimeter level, and the predicted self-gravity increments at the TMs are stable. This converts high-fidelity computations into a lightweight surrogate model suitable for online evaluation and reveals the coupling between the spacecraft CG and SG.