Abstract: Cement-soil deep mixing columns was used to improve the surrounding soils of drilled cast-in-place piles in a typical saturated silt site in Suqian City. Full-scale horizontal cyclic loading tests on cast-in-place piles on site were conducted. The hysteresis characteristics, bending moment, stiffness degradation, displacement ductility, horizontal bearing capacity, and residual deformation capacity of test piles in improved soil (TPI) and test piles in unimproved soil (TPU) under cyclic horizontal loads were explored. The test results show that the bending moment of TPI piles is reduced by 53%, the yield displacement is reduced by 41%, the horizontal bearing capacity is increased by 150%, the effective stiffness is increased by 240%, the equivalent viscous damping ratio is increased by 233%, the cracking load of the pile is increased by 150%, and the residual deformation capacity of the pile during the yield load stage is reduced by 17%. The cement soil mixing method can not only limit the liquefaction of silt at pile side, increase the energy consumption and displacement ductility of pile foundation, but also significantly reduce the development of horizontal displacement and bending moment of pile head. From the perspective of seismic performance, the reinforcement of existing pile foundations with cement soil mixing piles effectively suppresses the stiffness attenuation of the foundation soil within the reinforcement range, enhances the total energy consumption, equivalent viscous damping coefficient, and deformation recovery ability of the pile soil structure, and improves the seismic performance of the pile foundation.