Abstract: To explore the piping erosion mechanism and strength change characteristics of gap-graded cohesionless soil with different dry densities under the same stress condition, three kinds of soil samples with the dry densities of 1.80, 1.84 and 1.88 g/cm³ are prepared, and the effects of the dry densities on the disruption of the critical hydraulic gradient, cumulative sand gushing, volume change and strength change after erosion are studied through the piping triaxial tests. The study shows that the disruption of the critical hydraulic gradient of the samples increases with the increase of the dry density, and the disruption of the critical hydraulic gradient of seepage is analyzed by using the Terzaghi formula (F1), Mao Changxi formula (F2) and China Institute of Water Resources and Hydropower Research formula (F3). It is found that the calculated results by them are close to the experimental results in this stress environment (the degree of advantages is F2 > F3 > F1). The cumulative sand gushing amount and volume change of the samples show a negative correlation with the dry density, and its sand gushing rate gradually attenuates with the continuous development of piping erosion. The strength of soil is affected by both the dry density and the erosion rate, that is, the degree of strength reduction of soil increases with the erosion and decreases with the increase of the dry density.