In　this　paper,　an　auxiliary-model　method　is　proposed　for　calculating　equivalent　input　seismic　loads　in　research　of　ground　motions.　This　method　can　be　used　to　investigate　the　local　effect　of　3D　complex　sites　subjected　to　obliquely　incident　SV　and　P　waves.　Using　this　method,　we　build　a　fictitious　auxiliary　model　along　the　normal　direction　of　the　boundary　of　the　area　of　interest,　with　the　model＇s　localized　geological　features　remaining　the　same　along　a　vector　normal　to　this　boundary.　This　model　is　divided　into　five　independent　auxiliary　models,　which　are　then　dynamically　analyzed　to　obtain　the　equivalent　input　seismic　loads.　Unlike　traditional　methods,　in　this　new　technique,　the　mechanical　behavior　of　the　auxiliary　model　can　be　nonlinear,　and　its　geometry　can　be　arbitrary.　In　addition,　a　detailed　description　of　the　steps　to　calculate　the　equivalent　input　seismic　loads　is　given.　Numerical　examples　of　incident　plane-wave　propagation　at　uniform　sites　with　local　features　validate　the　effectiveness　of　this　method.　It　is　also　applicable　to　elastic　and　non-elastic　problems.