The transient stratification of the corium in the lower plenum and its impact on the heat flux distribution on the outside of reactor vessel is analyzed in this work. A method for predicting the kinetic corium pool structure is proposed, which takes into account both thermo-chemical equilibrium and density evaluation of the corium. The transient stratification of the corium pool formed after a large loss of coolant accident (LLOCA) and a station blackout (SBO) accident of ACP1000 nuclear power plant in China was analyzed by this method. The transient structure of the corium pool was calculated at the moment when the amount of molten materials in the corium pool increased obviously. The results shown that the formation of a three-layer pool is highly possible when a two-layer pool is formed in the previous moment with a heavy metal layer on the bottom and the density of the heavy metal layer at the bottom is greater than the density of the newly added molten material at the next moment. The heat flux on the outside of the vessel wall faced the thin top metal layer and the vessel failure probability of the vessel here are high if a three-layer pool occurred.