Based　on　the　soil　mechanic　model　in　saturated　porous　media,　by　combining　viscous-spring　artificial　boundary　with　the　2-D　explicit　finite　element　method,　an　explicit　numerical　wave　method　is　proposed　to　analyze　the　effect　of　the　local　terrain　on　the　dynamic　response　of　saturated　site　under　diffracting　earthquake　wave　field.　Then,　a　2-D　semicircular　valley　model　is　used　to　analyze　the　topographical　effect　on　the　diffracting　earthquake　wave　field　in　the　saturated　site.　A　comparison　is　made　between　the　numerical　solution　and　the　far　boundary　solution.　And　the　result　indicates　that　the　proposed　method　is　correct　and　applicable.