Internal erosion of soil is a major factor leading to engineering hazards such as dam instability and roadbed collapse. A modified GDS triaxial apparatus with a particle collection system was developed to monitor the real-time changes in flow rate, fine particle loss, and volumetric strain during the erosion process of silty sand. The effects of hydraulic gradient, confining pressure, and fines content on the erosion mechanism and post-erosion mechanical properties were systematically investigated. Results show that the erosion process exhibits distinct "clogging-loss" dual stages: the permeability coefficient initially decreases due to fine particle migration and clogging, followed by continuous loss of clogged particles leading to a recovery and eventual increase beyond the initial value. The increments in permeability coefficient, fine particle loss rate, and volumetric strain all increase with increasing hydraulic gradient and fines content but decrease with increasing confining pressure. Additionally, under low hydraulic gradient or high confining pressure conditions, the accumulation of migrating fines in the voids between coarse particles temporarily enhances the initial stiffness and strength of the soil. However, this strengthening effect diminishes with increasing strain, resulting in an overall decrease in soil strength. Furthermore, it is confirmed that the void ratio changes induced by erosion are the key factor affecting soil deformation characteristics, with a positive correlation between void ratio increment and maximum volumetric compression. The research results reveal the evolution patterns of internal erosion in silty sand and its effects on mechanical properties, providing theoretical basis for assessing the risk of erosion-induced engineering hazards.