Abstract: Capsule expansion technique (CET) is a new active deformation control technology, but its mechanism of pile foundation deformation control has not been thoroughly studied. In this study, numerical analysis is used to explore the control effect of capsule expansion on pile horizontal deformation. The deformation characteristics of pile and soil and the interaction between capsule-soil-pile were further analyzed. The results show that the pile can produce a maximum horizontal deformation of 5.5mm induced by CET with expansion diameter of 0.5m, and the control efficiency is 60%. CET would lead to a large excess pore water pressure of the adjacent soil. The dissipation of excess pore water pressure would cause the soil to be compressed under the additional stress and appear in reverse movement. When the expansion distance is small, the pile diameter presents a significant impact on the capsule expansion. The expansion has certain deformation control on piles with a diameter of 0.4~1.6m, especially suitable for deformation control of small diameter pile foundations. As the expansion diameter increases, the maximum horizontal displacement of the pile foundation increases almost linearly. In addition, the horizontal displacement of the pile decreases as the expansion distance increases. However, due to the limited influence of expansion on the surrounding pore water pressure, the control efficiency shows an increasing trend. The "blocking effect" and "reaction effect" are obvious in the deformation control of double row capsule expansion. The principle of "far first and then near, row by row expansion" should be followed to improve the control effect in controlling pile foundation deformation.