Abstract: The track structure method was used to model the early physical and chemical event from the incidence of monoenergetic electrons to DNA solution to the production of final DNA damage. The judgmental method of base damage induced by direct energy deposition, the exhaustive definition of classification of DNA damage and realization method on computer and the random sampling method for confirming the position of free radicals were introduced. The majority of interactions in DNA are in the form of no break, and most of the strand breaks are the SSBs which are easy to repair. In a small number of DSBs, the complex DSBs account for a considerable number of shares. The basic assumption that DNA is the main target of radiation reaction is validated.