<p>Seepage causes the soil structure changing that would influence the safety and stability of geotechnical infrastructures significantly. However, the effect of grain size distribution on seepage behaviors subject to suffusion of sand remains unclear to date. A series of suffusion tests on different grading considering the fines content and grading parameter was carried out to explore the seepage behaviors of gap-graded sand. The results showed that the suffusion evolution process of the gap-graded sand can be divided into three stages: stabilization, development, and failure stage. Fines content and grading parameter have a significant influence on the hydraulic conductivity of gap-graded sand. The critical hydraulic gradient, including initiation and failure hydraulic gradient, of different grading gap-graded sand can be obtained. The final fines loss rate decreases with an increase in fines content, and increases with an increase in grading parameter. The variation of global and local hydraulic conductivity of gap-graded sand can be described as a “tree structure”, which can be distinguished into two stages: “tree trunk” and “dendrites”. The local hydraulic conductivity of the upper parts is smaller than that of the lower parts of the sample. A three-dimensional surface describing the correlation among the fines eroded ratio, the suffusion time, and the initiation hydraulic gradient is proposed and verified by the tested data of sand and sandy gravel samples.</p>