This paper proposes a phenomenological model based on the resonant coupling of a local spacetime field, aiming to provide a unified explanation for the observed, unexpected residual velocity deviations (i.e., the “flyby anomaly”) during Earth flybys of spacecraft. The model assumes the existence of a local spacetime field around the Earth, modulated by its rotation and mass distribution, and decaying with distance. When a spacecraft passes through this field at a specific frequency, resonant coupling may occur, generating a tiny instantaneous acceleration pulse. The model naturally derives the three key features of the anomaly: spatial locality, stochastic occurrence, and weak but reproducible magnitude. By fitting the data from three typical flyby events (Galileo-1990, NEAR, Rosetta), the model obtains a self-consistent set of parameters. It further provides a reasonable explanation for the non-observed anomaly in the Cassini flyby and makes testable predictions for future flyby events (e.g., Juno) and possible laboratory analog experiments.