Settling velocity is a fundamental parameter in sediment transport dynamics.For uniform Par-ticles,there are abundant formulas for calculation of their settling velocities.But in natural fields,sedi-ment consists of non-uniform particles.The interaction among particles is complex and should not be neg-lected.In this paper,based on the analysis of settling mechanism of non-cohesive and non-uniform parti-cles,a theoretical model to describe settling mechanism is proposed.Besides suspension concentration andupward turbulent flow caused by other particles,collision among particles is another main factor influ-encing settling velocity.By introducing the collision theory,equations of fall velocity before collision,colli-sion probability,and fall velocity after collision are established.Finally,a formula used to calculate the set-tling velocity of non-cohesive particles with wide grain gradation is presented,which agrees well with theexperimental data. Settling velocity is a fundamental parameter in sediment transport dynamics. For uniform Par-ticles, there are abundant formulas for calculation of their settling velocities. But in natural fields, sedi-ment consists of non-uniform particles. The interaction among particles is complex and this paper, based on the analysis of settling mechanism of non-cohesive and non-uniform parti-cles, a theoretical model to describe settling mechanism is proposed .esides suspension concentration andupward turbulent flow caused by other particles , collision among particles is another main factor influ-encing settling velocity. By introducing the collision theory, equations of fall velocity before collision, colli-sion probability, and fall velocity after collision are established. Finally, a formula used to calculate the set- tling velocity of non-cohesive particles with wide grain gradation is presented, which agrees well with the experimental data.