Abstract: The "Code for Seismic Design of Urban Bridges" assumes synchronous vibration between pile cap and the ground, ignoring the lateral soil resistance of the pile cap when calculating the seismic design force for low-pile cap foundations based on the capacity protection design principle. The "Standards for Seismic Design of Bridges" in Shanghai considers the contribution of lateral soil resistance of the pile cap on this basis. However, the applicability of this method to soft soil foundations has yet to be demonstrated. Therefore, this study focuses on typical pile foundation bridges in Shanghai, considering the nonlinear dynamic soil-pile and soil-cap interaction. A nonlinear finite element model of pile foundation bridges in soft soil is established, and incremental dynamic time-history analysis is conducted. This analysis investigates the variation of seismic inertia force of the pile cap and lateral soil resistance with pier height and seismic intensity, revealing the force transfer mechanism from the pier to the bottom of the pile cap under seismic excitations. Based on this, a new simplified calculation method for the seismic design force of pile group foundations is proposed using the capacity design method. Compared with the current codes, the new simplified calculation method significantly improves the calculation accuracy of design seismic shear force and bending moment for pile foundations in soft soil, while having a certain margin of safety.