Abstract: To investigate the variable-mass permeability of pebble soils, a large-scale test system for particle migration was designed and developed. The variable-mass seepage tests were conducted on continuous multi-graded pebble soils to examine the effects of giant- and fine- grained contents on particle loss, permeability and erosion modes. The results indicate that: (1) The erosion process can be divided into three stages: intense erosion, erosion deceleration and stable seepage. The Reynolds number (Re) and permeability coefficient continuously increase with the duration of erosion, and the flow regime transforms from Darcy to turbulent flow. (2) The effect of giant-grained content exhibits a threshold-dependent behavior, with three critical values (18.3%, 38.2%, and 66.7%). The exceeding of each threshold triggers substantial modifications in soil-rock contact, pore structure and filling density. As the fine-grained content increases, the eroded mass of pebble soils first increases and then decreases, while the permeability coefficient decreases in a “step-like” pattern. (3) The seepage erosion modes of pebble soils include localized erosion at the initiation point, progressive expansion erosion and overall penetration erosion. Driven by the variable-mass seepage, the skeleton-pore structure and permeability vary synchronously, showing the collaborative evolution characteristics of permeability coefficient increase and flow regime transition.