Abstract: The cement-soil mixing columns are used to improve the surrounding soils of drilled cast-in-place piles in a typical saturated silt site in Suqian City. The full-scale horizontal cyclic loading tests on the cast-in-place piles on site are 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 those in unimproved soil (TPU) under horizontal cyclic loading are explored. The test results show that the bending moment of TPI piles decreases by 47% and the yield displacement decreases by 41% compared to those of TPU piles. The horizontal bearing capacity of TPI piles increases by 150%, the effective stiffness increases by 240%, and the equivalent viscous damping ratio increases by 233%. The cracking load of the piles increases by 150%, and the residual deformation capacity of the piles during the yield load stage decreases by 17%. The cement-soil mixing method can limit the liquefaction of silt at pile side, increase the energy consumption and displacement ductility of pile foundation, and significantly reduce the development of horizontal displacement and bending moment of pile head. From the perspective of seismic performance, strengthening the existing pile foundations with cement-soil mixing piles can effectively suppress the stiffness attenuation of the foundation soils within the reinforcement range. The cement-soil-reinforced piles enhance the total energy consumption, equivalent viscous damping coefficient and deformation recovery capability of the pile-soil structure. Therefore, the seismic performance of pile foundations during large deformation is improved.