Abstract: It is the sand particle gradation and the sediment mineral composition which affect most the measurement of the suspended sediment concentration with the turbidity method. The field non-cohesive sediment of the Yellow River was used to prepare sandy water with different particle sizes and suspended sediment concentrations. Based on the group of experiments on fixed-size particles of sediment, coarse and fine particles of sediment, and mixed particles of sediment, the influence of the sediment gradation on the measured turbidity of Optical Back Scatteering (OBS) was quantitatively analyzed, and the influencing mechanism was preliminarily discussed. The results show that under the condition of fixed-size particles of sediment, the influence characteristic of non-cohesive sediment particle size on the water turbidity conforms to the Mie scattering law. The magnitude of the impact of the suspended sediment concentration on OBS output turbidity is in the order of 10-10², while the impact of the sediment particle size is in the order of 10⁻¹-1. The relationship of the specific surface area of the suspended sediment and the measured turbidity of OBS accords with linear distribution in the same sediment concentration condition. A formula between the sediment concentration and the OBS output turbidity is established considering the influence of non-cohesive sediment gradation to accurately reflect the impact of non-cohesive sediment particle size differences and sediment concentration changes on water turbidity. This work can provide reliable evidence not only for the accuracy of the calculation of the sediment concentration and water turbidity, but also for the calibration of the measurement results.