Abstract: Establishing the relationships among the erosion process, development mechanism and macroscopic phenomena by exploring the movement of soil particles under the local flow field is one of the important methods to study the mechanism of scour. Four sand samples with particle sizes of 0.075, 0.25, 0.5 and 2.0 mm are tested in laboratory by using the improved test device based on the simplified scour resistance tests (SSRT). The erosion depth, deposit height and deposit range of the samples under the local flow field generated by the blade at the speed of 50,70 and 90 r/min are recorded, and the influences of local flow field and particle size on sediment transport are analyzed. According to the different characteristics of sand particles under the flow condition, sand samples can be divided into three categories: non-erodible sand, sand which is erodible and easy to deposit, and sand which is erodible but not easy to deposit, with the calculated "universal start" velocity and the critical velocity obtained from the laboratory tests, so as to judge the scour resistance of soil. On this basis, the concept of soil particle retention ratio is defined based on the proportion of soil volume in the erosion area and the deposit area, and the quantitative relationship between the critical velocity and the "universal start" velocity is established.