Abstract: To investigate the seismic response characteristics of composite foundation of concrete core mixing pile in marine-terrestrial interaction soft soil areas, the Jiangji Section project of the Zhuzhao High-Speed Railway has been selected as a representative case. A modified Davidenkov constitutive model incorporating a critical shear modulus has been developed to capture the nonlinear dynamic behaviors of soft soils more accurately. Considering site stratigraphy and pile-soil interaction, three representative strong motion records are adopted as input ground motions. A three-dimensional nonlinear finite element model of the railway composite foundation is established using ABAQUS for seismic response analysis. The results indicate that: (1) The proposed modified Davidenkov model realistically reproduces the soil’s dynamic stress-strain responses; (2) Significant site nonlinearity is observed under different earthquake intensities, with notable amplification of surface acceleration. Both acceleration and displacement decrease with increasing depth, and the Chi-Chi ground motion induces the largest pile responses and surface settlement, representing the most unfavorable seismic scenario; (3) A sensitivity analysis of parameters reveals that optimal seismic performance and cost-effectiveness are achieved when the length ratio of core pile to cement-soil column is 0.6-0.8, the diameter ratio of cement-soil to core pile is 2.0-2.5, and the shear modulus ratio of cement-soil to soft soil is 30-45. These findings provide theoretical supports and engineering references for the seismic design and optimization of similar composite foundations in soft soil regions.