Abstract: Field immersion test is an important means to study the characteristics of self-gravity collapsibility of deep loess site and evaluate its collapsibility. Based on physical and mechanical parameter tests of undisturbed loess, field immersion test was carried out in deep loess site in Lanzhou new district to test and analyze the development law of water content of soil layer and foundation settlement deformation, study the self-weight collapsible characteristics of the deep loess site, and evaluate the self-gravity collapsible subsidence of this deep loess site. Researches show that the moisture content and dry density of the undisturbed loess in Lanzhou new district increase gradually with the depth, but the porosity ratio and gravity collapsibility coefficient decrease greatly. With the increasing of water infiltration depth, the growth time of volumetric water content curve increases successively, and the time of wetting front reaching the measuring point lags more and more. The surface subsidence experiences a slow-fast-slow down-steady process, and the subsidence rate can reach 5.88 cm/d. 60% of the total subsidence is completed among 50 d. The depth range of 0~16.5 m is the soil layer with strong subsidence, and its subsidence accounts for more than 80% of the whole site. A large number of circumferential fractures developed around the test pit, with the widest width of 34 cm and the height of stage-off up to 50 cm. Based on the laboratory and field tests, 24 m can be used as the lower limit of gravity collapsibility depth and foundation treatment depth of the site. The research results can provide a reference for engineering construction in deep collapsible loess area.