Abstract: The surge-root irrigation supporting casing has significant effects on its outflow process. In order to find out the optimal pore size of PP cotton as a supporting casing for surge-root irrigation in apple on the Loess Plateau, an indoor soil box test was carried out on the example of Yan’an yellow sandy soil in Shaanxi Province, and a comparative experimental design was used to analyze the effects of different pore sizes of PP cotton（1 μm, 10 μm, 20 μm and 50 μm） on the soil moisture infiltration characteristics when used as a casing for surge-root irrigation. The results are as follows. Under different PP cotton pore sizes, the shape of the wetting body formed by water infiltration was approximately ellipsoidal, and the transport distance of the wetting front in each direction was in line with the power function relationship with the infiltration time（P＜0.05）. By comparing the characteristics of the wetting body with different PP cotton pore sizes, it was determined that when the pore size of the PP cotton was 50 μm, the transport distance of the wetting front of the wetting body, the wetting body volume, and the region of high water content within the wetting body were all the largest, and the redistribution process was also the largest, and the redistribution process was the largest. In the redistribution process, the distribution of soil moisture in the wetted body is also the most uniform one. The results of the study can provide theoretical basis for the selection of casing parameters, which is of great significance for the large-scale popularization of surge-root irrigation in apple orchards in the Loess Plateau region.