The invention discloses a loess self-weight collapse amount determining method. According to the method, an e-lgP compression curve of a soil sample at each depth layer is drawn according to an indoorconsolidation test result, and the early consolidation pressure of loess is determined according to the compression curve; under-consolidated loess is deduced to be self-weight collapsible loess according to the fact that the early consolidation pressure of loess in the saturated state is the initial collapsing pressure, and therefore the consolidation state of a soil sample is judged by calculating the over-consolidation ratio (OCR) of the soil sample of each depth layer, and the total self-weight collapse amount of loess is obtained by calculating the collapse amount of the under-consolidated soil sample. Compared with a traditional indoor compression experiment, the measurement error with between the indoor test value and an outdoor measured value is greatly reduced, the measurement precision of the indoor loess self-weight collapse amount is improved, and a new way is provided for determining the loess self-weight collapse amount.