Abstract: This article discusses the dynamic computation of the closed cylindrical shell under impact load. In the text we analyse the changes of the momenta and the energy on each stage in the impact process, take into account the effect of the mass of impact object and the system of the closed cylindrical shell by impact, and transform the distributed mass of the whole cylindrical shell into an only concentrated "equivalent mass" by the method of reduced mass. Consequently we derive the dynamic factor of the closed cylindrical shell due to impact load.The method proposed in this paper is of practical worth and is more convenient in calculations.