<p>The gap-graded granular soil is prone to erosion under the influence of water infiltration. The change in soil particle distribution and the erosion-affected mechanical properties have a significant impact on the stability of geotechnical engineering. In order to quantify the impact of internal erosion on the mechanical behaviors of gap-graded granular soil, the state-dependent multi-mechanism bounding surface model is modified by correlating the percentage of fine particle loss to the critical state of the gap-graded granular soil in this work. This modification allows the model to provide a quantitative description of the variations of bounding surfaces and lines with varying degrees of internal erosion, and thus the mechanical behaviors of the eroded soil such as the peak strength, shear stiffness, and volume expansion potential can be simulated. A series of triaxial and direct shear tests are conducted on eroded quartz sands to validate the developed constitutive model and further investigate the stress–strain behavior of the sands after internal erosion. These findings indicate that the developed constitutive model is capable of characterizing the variations of stress–strain relationships of gap-graded granular soils due to internal erosion. Internal erosion tends to diminish the peak strength, shear stiffness, and volume expansion potential of the eroded sand samples. A single exponential decay function is proposed to quantify the relationship between the peak friction angle and the percentage of fine particle loss to facilitate engineering applications.</p>