When　applying　the　three-dimensional　discontinuous　deformation　analysis　(3D-DDA)　method　to　simulate　the　dynamic　response　of　rock　mass　in　an　earthquake,　appropriate　artificial　boundaries　and　input　methods　should　be　considered.　In　this　study,　the　3D-DDA　method　was　modified　by　introducing　the　viscoelastic　artificial　boundary　and　the　seismic　motion　input　method　based　on　the　wave　field　decomposition　method　and　with　the　consideration　of　the　lateral　free-field　wave.　Subsequently,　the　correctness　of　the　boundary　and　the　input　method　was　verified　through　numerical　examples.　The　results　of　the　numerical　examples　were　almost　consistent　with　the　theoretical　solution.　The　introduced　viscoelastic　artificial　boundary　and　seismic　motion　input　method　improved　the　applicability　of　the　3D-DDA　method　and　can　be　further　applied　to　simulate　actual　engineering　cases.　Viscoelastic　artificial　boundary　is　applied　and　validated　in　3D-DDA.The　differences　between　viscoelastic　boundary　and　existing　boundaries　are　compared.Modified　3D-DDA　can　consider　wave　field　decomposition　and　lateral　free-field　wave　for　seismic　motion　input.Feasibility　of　modified　3D-DDA　to　study　the　dynamic　response　of　rock　mass　in　an　earthquake　is　studied.