Abstract: Collapsibility of loess is inconstant. Effects of moistening and demoistening, caused by obvious variety of water content of loess, influence strongly to collapsibility of loess. When proceed to the evaluation to collapsibility of loess, should pay attention to synthetically analyse and estimate disadvantageous influences, caused by effects of moistening and demoistening, with the dynamic point of view and method, in order to adopt to effective measure to dissolve hidden trouble to engineering.