Structural　finite　element　coupled　with　the　conjugated　infinite　element　method　is　an　efficient　numerical　technique　for　solving　the　acoustic　radiation　problem　due　to　the　vibration　of　underwater　objects.　However,　for　large　complex　structures,　the　total　acoustic　mesh　would　become　very　large　if　the　artificial　boundary　is　too　far　away　from　the　structural　wetted　surface.　Thus,　the　calculation　time　can　become　too　long　to　confine　the　application　of　the　conjugated　infinite　element　method.　On　the　other　hand,　if　the　artificial　boundary　is　close　to　the　structural　wetted　surface,　it　will　lead　to　computation　accuracy　losing　due　to　the　near-field　effects.　Consequently,　it　is　essential　to　present　some　guidelines　based　on　the　physical　mechanism　of　structural　acoustics　to　choose　a　suitable　artificial　boundary　that　optimizes　calculation　accuracy　and　efficiency.　In　present　work,　the　evanescent　wave　theory　of　an　infinite　length　cylindrical　shell　is　adopted　to　theoretically　analyze　the　decay　characteristic　of　evanescent　waves　in　near　field.　Then,　the　effect　of　the　position　of　artificial　boundary　on　computation　accuracy　of　conjugated　infinite　element　for　a　finite　length　ring-stiffened　cylindrical　shell　is　numerically　investigated.　Results　suggested　that　for　the　cylindrical　shell　mentioned　in　this　study,　the　artificial　boundary　can　be　placed　at　least　0.4　times　the　acoustic　wavelength　away　from　the　structural　wetted　surface.　What＇s　more,　for　high　frequencies　or　large-scale　structures,　the　required　non-dimensional　distance　between　the　artificial　boundary　and　the　structural　wetted　surface　increases.