Abstract: To simplify the determination of pile foundation internal forces in high-pile wharves under bidirectional horizontal seismic action, this study proposes a calculation method whereby the internal forces under lateral seismic action are multiplied by an increase coefficient to obtain the total internal forces. To derive an analytical expression for the increase coefficient, the actual discrete pile foundation of the wharf is approximated as a continuous pile foundation. This approach considers the simultaneous action of bidirectional horizontal seismic inertia forces along the two principal axes of the wharf and is based on the conditions of static equilibrium. To verify the rationality and accuracy of the proposed calculation method, a time history analysis was conducted using 20 sets of bidirectional horizontal seismic records. This analysis considered wharves with different length-to-width ratios and eccentricities to determine the corresponding internal force increase coefficients, which were then compared with the results from the analytical expression. The study indicates that the calculation formula for the internal force increase coefficient can be expressed as a function of the length-to-width ratio and eccentricity of the wharf. This method offers a generally applicable and precise alternative to the complex seismic response analysis under bidirectional horizontal seismic motions.