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, sediment consists of non-uniform particles. The interaction among particles is complex and should not be neglected. In this paper, based on the analysis of settling mechanism of non-cohesive and non-uniform particles, 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 influencing settling velocity. By introducing the collision theory, equations of fall velocity before collision, collision probability, and fall velocity after collision are established. Finally, a formula used to calculate the settling velocity of non-cohesive particles with wide grain gradation is presented, which agrees well with the experimental data.