Abstract: In order to study the deformation characteristics of collapsible loess foundation in South Shanxi, a series of laboratory and in-situ tests which included three in-situ ponding tests (12m × 12m, 15m × 15m and 20m × 20m) and three in-situ ponding plate loading tests were carried out. The results from ponding tests showed that the type of collapse in-situ indicated in the engineering investigation report was erroneously judged and the site should be non-self-subsiding. The regularities of collapsible deformation of loess foundation and the evaluation of the type of collapse suggested in the Chinese Regulation on Building Construction in Regions of Collapsible Loessic Soils (TJ25-78) are discussed, and a modified coefficient m,, is suggested. By using fuzzy mathematics, a synthetic judgement for sensitivity to self-subsidence is given, and an index as well as the sensitivity criterion is also proposed. This method is easy and has high reliability. The collapsible deformation of collapsible loess foundation is calculated by non-linear finite element method. The calculated values agreed with the measured values.