In order to reveal the characteristics of sediment deposition in trapezoidal channels under different inflow conditions, a scale model test method was used to study the characteristics of sediment deposition morphology and particle size distribution in channels under different flow rates, sediment concentrations, water depths and sediment gradations. It was found that variations in flow rate influence flow velocity, sediment transport capacity, and secondary flow intensity, resulting in a dynamic evolution process of sediment deposition characterized by uniform settling, localized erosion, and complex deposition patterns. Changes in water depth alter flow dynamics and sediment transport mechanisms, thereby determining the complexity and diversity of deposition behavior. An increase in sediment concentration enhances inter-particle interactions, leading to a transition in deposition pattern from uniform suspension to localized scouring. Variations in sediment gradation affect particle settling behavior and the flow’s ability to transport sediment, consequently modifying the height and shape of deposited sediments. Under the most sedimentation-prone conditions, characterized by low flow rate (≤2.635 m³/s), high sediment concentration (≥7.5 kg/m³), coarse sediment gradation (mean particle size: 0.548 mm), and elevated water depth (1.2 m), the flow velocity and sediment transport capacity are measured at 0.813 m/s and 0.286 kg/m³, respectively. The maximum deposition thickness reaches 38.948 cm, exceeding the deposition thickness observed under low-concentration conditions (4.65 kg/m³) by more than 1.3 times. The results can provide references for the design, operation and anti-siltation of trapezoidal channels under sediment-laden flow conditions.