Abstract: The water infiltration in loess slopes is of great significance for the change of physical and mechanical properties of soil. There are many preferential channels in loess and the study on the preferential flow is crucial for the understanding of water infiltration process in loess. The distribution of fractures especially the microscale fractures is detected by using the electrical resistivity tomography (ERT) and geological radar. Furthermore an in-situ single-ring infiltrometer test is carried out in the hidden fracture areas to monitor the infiltration process. Based on the in-situ test results, a single-permeability model and a dual-permeability model are set up to simulate the infiltration process in the preferential channel. The conclusions can be drawn as follows: (1) The hidden fractures in loess are significant for the preferential flow which is always ignored in the engineering. (2) The in-situ infiltration process in preferential flow channels is revealed: the water infiltrates into the fractures first, then the unsaturated infiltration process begins, and water infiltrates downwards uniformly, at the same time the water in the preferential flow channels spreads into the matrix flow area. (3) The numerical method simulates two types of irrigation conditions: high intensity with a short duration and low intensity with a long duration. For the high intensity case, the preferential flow dominates and it has a positive effect on the infiltration, and the irrigation water infiltrates quickly into the deep soil and induces the increase of groundwater with the top matrix domain unsaturated. For the low intensity case, the matrix flow dominates and it has a saturated infiltration state.