Abstract: The capsule expansion technique (CET) is a new active deformation control technology, but its mechanism of deformation control of pile foundation has not been thoroughly studied. Numerical analysis is performed to explore the control effects of capsule expansion on horizontal deformation of piles. The deformation characteristics of piles and soils and the interaction among the capsules, soils and piles are further analyzed. The results show that the maximum horizontal deformation of 5.5 mm, is induced by the CET with expansion diameter of 0.5 m, and the control efficiency is 60%. The CET leads to a large excess pore water pressure of the adjacent soils. The dissipation of the excess pore water pressure causes the soils to be compressed under the additional stress. When the expansion distance is small, the pile diameter presents a significant impact on the capsule expansion. The expansion has certain deformation control on the piles with a diameter of 0.4~1.6 m, 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 influences 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 the double-row capsule expansion. The principle of "far first and then near, row by row expansion" should be followed to improve the control effects in controlling deformation of pile foundations.