Abstract: Equipment will cause spectrum dip effect under underwater explosion. In order to study the influence of installed equipment on shock environment, the impedance of basement and equipment were calculated by frequency response analysis of ABAQUS. And then the relationship between basement response and impedance was obtained by a simplified dynamic model of equipment-basement. Based on the difference between steady state response and transient response, a reduction formula to describe how installed equipment affects shock environment of basement was developed. The results show that the reduction formula based on impedance better agrees better with experiment data in most areas than other reduction formulas.