Based　on　the　dynamics　of　flexible　multi-body　system,　a　dynamic　model　for　underactuated　robot　with　flexible　links　was　presented　using　assumed　mode　method.　In　this　model,　the　structural　characteristics　of　underactuated　robot　were　considered.　The　solution　to　this　model　was　analyzed　and　the　various　boundary　conditions　of　modal　function　at　the　passive　joints　were　discussed.　The　simulation　was　given　via　position　control　of　the　passive　joints　of　a　2DOF　underactuated　manipulator.　The　results　demonstrate　the　elastic　deformations　of　the　flexible　links　and　reflect　the　vibration　characteristics　of　the　system　at　different　actuator　placements,　which　verify　the　efficiency　of　the　proposed　model.　The　simulation　also　reveals　that　it　is　necessary　to　control　the　vibration　of　flexible　links.