Abstract: The ion irradiation will cause different degrees of radiation damage to the target material. One of the important physical parameters to evaluate the radiation damage is dpa, that is, the displacements per atom. The Monte Carlo method based SRIM simulation program, which describes the collision and energy loss between ions and the target, is widely used to calculate many parameters related to particle beam irradiation. The number of atomic shifts per unit depth per incident particle can also be calculated as an important parameter in dpa. In this paper, the single element target material is simulated by two common models of SRIM program. The NRT mathematical model based on damage energy is used to calculate the number of displacements indirectly and the output file is used to get the displacements number directly. The results show that for the single element target, the displacements calculated by the mathematical model are basically the same under the two modes of SRIM Quick damage and SRIM Full cascade, which can be used to calculate the dpa reliably. The displacements obtained directly from the output file of the two modes have about two-fold difference. To get the reliable dpa parameters, it is necessary to select the appropriate calculation mode according to different irradiations.