Abstract: In the Loess Plateau of the Yellow River Basin, loess disasters are frequent, and the key geologic gene is that the loess is extremely sensitivity to water. Among them, the scientific evaluation of wetting-induced collapse deformation of loess site is one of the themes. The wetting-induced collapse deformation of loess site gradually changes from "maximum potential" to "possible potential", and there is an urgent need to find out the multi-field coupling process of water infiltration, gas migration, and wetting-induced collapse deformation in loess during the on-site pit immersion test. At present, there are few reports about the measured pore gas pressure and its evolution law during the on-site pit immersion test. Accordingly, the site immersion test in loess plateau site on large thickness of self-weighted wetted loess were carried out. Moisture sensors and pore pressure sensors were arranged at different depths of typical profiles, to directly measure the water transport and pore gas pressure during the water immersion process, and thus then reveal the water transport and gas pressure formation laws. The results show that the 5-stage change rule of perception of water infiltration - wetting - saturation -drying - stabilization during the on-site pit immersion after stoping. The installation of water injection holes has obviously changed the water diffusion paths in the pit, and the radial seepage from the injection holes and the enrichment of water near paleosoil stratigraphy, whereby resulting in a predominantly upward migration of water. Having realized for the first time, the in-situ measurement of pore gas pressure of the soil during soaking in a large test pit on large thickness of the collapsed loess site. It showed fluctuating changes, and the formation mode of the pore gas pressure during soaking was proposed. The results provided experimental evidence of the coupled water-gas transport in unsaturated loess, and lay the foundation for further refinement of the prediction of the wetting-induced collpase of the large-thickness loess.