The work aims to analyze the characteristic velocity model of the debris cloud by focusing on the hypervelocity impact of a disk projectile on a thin plate. The difference in particle velocity under isentropic unloading of shock waves and sparse waves for disk projectile material Cu-OFHC and thin plate material Al 6061-T6 under different impact pressure was calculated, and the effect of isentropic unloading of sparse waves on the characteristic velocity of the debris cloud under different unloading pressure was studied. The sparse wave isentropic unloading theory was used to solve the characteristic velocity of the debris cloud, and the Bless model characteristic velocity was modified to obtain an isentropic correction model for the characteristic point velocity of the debris cloud caused by the hypervelocity impact of a disk projectile on a thin plate. Compared with the calculation results of the Bless model, the isentropic unloading modified model shows better accuracy in predicting the experimental results, especially in predicting the velocity of the debris cloud head under higher velocity impacts.