Abstract: The immersion tests on a self-weight collapse loess site are conducted by setting water injection holes in Jinzhong City of Shanxi Province, China. A method for measuring the horizontal displacement of underground loess outside the test pit is proposed. The collapsible deformation of ground surface and underground, the influence region of loess collapse, the development process of fissures around the test pit and the underground lateral deformation are monitored and studied. The computing method of area correction factor is discussed. The results indicate that the curve of the collapsible process includes 6 stages. The influence region of the loess collapse relates to the thickness of the self-weight collapsible loess, and their ratio is about 1.6, which is obtained from the results of 4 immersion tests in 4 provinces. A new computing method based on the weighted average of the area that stands for different collapse volumes is established, and then the area correction factor is calculated as 0.7, which is greater than the value given by the Chinese Code. The development process of the fissures around the test pit experiences 4 steps, and the thickness of the self-weight collapsible loess determined by the immersion tests is 18 m, which is less than the value got from the indoor soil tests. The shallow loess outside the test pit moves inside while the deep loess moves outside, and the depth of inflection point decreases when the distance between the monitoring line and the test pit increases. All the results have been applied to the design of foundation treatment in later construction projects in this site, and they can also guide other projects in nearby regions.