To address the deficiencies of traditional diffusion models, a mixing length coefficient model was introduced to establish a vertical distribution model of suspended sediment. The effects of different factors on the vertical distribution of suspended sediment were investigated. The results show that when the actual mixing length coefficient is smaller than the commonly assumed value of 0.4, the sediment concentration calculated using the assumed value is overestimated, and vice versa. An increase in sediment concentration intensifies particle interactions, and the vertical distribution of sediment concentration presents a characteristic of “smaller at the upper part and larger at the lower part”. Larger particle size results in greater sediment settling velocity, causing more sediment to accumulate near the bottom and making suspension more difficult. Higher flow velocity enhances vertical turbulence, reducing the sediment concentration gradient between the riverbed and the water surface.