Abstract: A series of triaxial tests with σ3/σ1 = constant and confined compression tests were carried out. Some laws of collapsible deformation were found; Stress state and stress ratio have great influence on the collapsible deformation; spherical and shearing stresses have crossing effects on it. It is demonstrated that collapsible deformation calculated with δs is not propriety, therefore, the paper proposes that when the collapsible deformation of loess foundation is calculated the foundation must be divided into various zones according to stress state then the collapsible strain is calculated with corresponding parameters in each zone. Finally an example is presented to illustrate the calculation with parameters abtained in triaxial test. The results of calculation agree well with the in situ tests of Lanzhou and Shanxi.