The marine exploration environment is generally regarded as a fluid-solid boundary coupling medium. High-precision imaging of the fluid-solid boundary coupling medium can be achieved by using the decoupled elastic wave field for reverse-time migration. Traditional elastic wave decoupling utilizes the Helmholtz principle, but the separated P-waves and S-waves have phase and amplitude issues, which seriously affect the final imaging quality. This paper starts from the first-order velocity-stress elastic medium wave equation, separates the full elastic wave field into pure P-waves and pure S-waves to construct a new coupling equation, avoiding the phase and amplitude problems and crosstalk artifacts caused by the traditional Helmholtz decomposition; analyzes the displacement and stress continuity conditions at the boundary of the fluid-solid boundary coupling medium, and establishes the acoustic-elastic coupling vector separation equation; compares the numerical simulation results with the acoustic wave equation to verify the effectiveness and accuracy of the proposed acoustic-elastic coupling vector separation equation forward modeling method, and compares the results with the conventional reverse-time migration imaging method in a complex seabed model to verify the reliability of the reverse-time migration imaging results.