Abstract: A large-scale field water immersion test has been carried out in Heping Town, Lanzhou, the self-weight collapsibility of the site has been measured in this paper, and the amount of collapsibility has been calculated by using the experimental data. The influence of immersion time and infiltration depth must be considered in the field water immersion test. When the wetting peak does not reach the bottom of collapsible soil, there will be a large part of residual collapsibility, which underestimates the actual amount of collapsibility. In this field immersion test, the depth of collapsible soil layer is 24m, the depth of wetting peak is about 12m, and the depth of wetting peak is about 12m. According to the estimation of average infiltration rate, it will take more than 120 days to reach the bottom of collapsible soil layer. Under the condition of full immersion of collapsible soil layer, the measured value of large thickness collapsible loess in Lanzhou area is much larger than the calculated value, and their ratio is about 2.0, which is greater than the correction coefficient of 1.5 given in the investigation specification. In conclusion, in the field immersion test, in addition to determining the landform unit of the test site and the thickness of the wet soil layer, a reasonable soaking time must be given in order to fully wet the collapsible soil layer. After flooding, the depth of wetting peak should be measured in time to calculate the actual collapsing amount of soil layer in order to obtain a more reasonable correction coefficient.