To prove salt transport in roadside soil of low roadbed highway, Heda highway (G11) low roadbed roadside soil was selected as the research object. The preferential flow principle was used to explore the salt transport in the soil water movement process. Analyzed soil profile preferential flow horizontal and vertical distribution characteristics by using brilliant blue dye tracing and image analysis method. The results showed that: Staining area and non-staining area of soil properties had significant differences, staining area had more pore. In the horizontal profile, the number of preferential flow paths with different influence radius decreased with the increase of soil depth. The number of preferential flow paths with 1.0~10.0 mm influence radius in the same layer were far more than the number of paths >10.0 mm influence radius. In the vertical profile, the preferential flow characteristic index as dye coverage (DC), uniform infiltration depth (UF), preferential flow fraction (PF-fr), length index (LI) and peak (PI) were 40.10%, 4.10 cm, 801.96 cm2, 74.44%, 543.6 and 5 respectively. There was obvious preferential flow in roadside soil of low roadbed highway which played an important role in the transportation of salt in soil. The proposed indices provided a good research method to quantify the occurrence degree of soil preferential flow in the future.