Abstract: This article discusses the problems of the dynamic computation for thin-walled structures such as thin plates and thin shells under impact load to find the dynamic factor mainly.In calculation we take into account the effect of the mass of the striking object and the system of thin-walled structures to be struck and transform the distributed mass of thin-walled structures into only one concentrated "equivalent mass" by the method of reduced mass.Accordingly we derive the dynamic factor for the system of thin-walled structures under impact load.