Abstract: For comprehensive understanding about the formation causes and mechanical mechanisms of various types of pit-collapses in the lower reaches of the Yangtze River, taking the pit-collapse in the Hechangzhou head as an example, deep analysis was done, and then the soil mass instability model and displacement estimation method were proposed based on the results of soil mechanics. The results show that the pit-collapse in the Hechangzhou head belongs to lateral-slide type. Its formation cause is a there is a silt layer easy to liquefy in the beach soil. When there is a certain external vibration disturbance factor, once the liquefaction condition is mature, the silt layer becomes a sliding layer, the liquefaction layer and the overlying soil layer would turn into large lateral slides. The slide soil mass would skid out of the collapse pond mouth into the deep channel groove, and then is quickly dispersed and transported to the downstream by the main river current. The empirical formula of power function between the distance of sliding soil mass and the thickness of liquefied layer and dip angle of sliding layer can be obtained by using mathematical statistics method. These results can provide a reference for the early warning and managing of such collapse in the lower reaches of the Yangtze River.