Upon infiltration, the mechanical behaviors of pile foundations in collapsible loess areas will deteriorate significantly due to the reduction of soil matric suction and collapsible deformation, which will lead to a series of engineering problems. In order to study the changes in the bearing behaviors of pile foundations in collapsible loess before and after infiltration, infiltration tests of model piles were carried out. The changes in the soil around the pile (including settlement, volumetric moisture content and matric suction) as well as the changes in the mechanical behaviors of pile foundations (including the pile head settlement, pile shaft friction and pile base resistance) were monitored. The changes in soil matric suction around the pile perimeter during infiltration were correlated with the changes in pile foundation bearing properties to elucidate the changes in pile foundation bearing properties before and after infiltration based on unsaturated soil mechanics. Research results show that the pile top settlement and pile end bearing capacity increase continuously during the infiltration process, and the pile axial force is roughly distributed in a "D" shape, with the maximum axial force appearing during the infiltration process. During the infiltration process, the matric suction of the loess around the pile decreases, resulting in collapsible deformation, which changes the magnitude and direction of the pile lateral frictional resistance, thus leading to changes in the bearing properties of the pile foundation. Based on the principle of unsaturated soil mechanics, the traditional shear displacement method is simplified and modified, and a single pile settlement prediction model is proposed to take into account the effect of collapsible deformation of the pile foundation. The theoretical model is validated with the experimental results. The research results of this paper help to further improve the design theory and engineering property evaluation of pile foundation in collapsible loess areas under complex environmental loads.