Urban　earthquake　disaster　prevention　is　regarded　as　an　integrated　systematic　engineering.　Urban　earthquake　disaster　prevention　system　is　made　up　of　all　the　earthquake　disaster　prevention　activities.　The　concept　and　composition　of　urban　earthquake　disaster　prevention　system　periphery　were　presented　based　on　system　periphery　theory.　A　seismic　risk-control　mechanism　model　of　system　periphery　was　deduced　using　exchange　rate　of　periphery　as　a　dependent　variable,　and　an　observability-controllability　model　of　system　periphery　was　established　and　crystallized　in　its　application　to　the　quantitative　analysis　of　practice　problem.　The　input　sets　of　urban　earthquake　disaster　prevention　system　are　determined　as　the　maximum　earthquake　magnitude　happened　in　or　around　the　city,　the　measurable　earthquake　frequency,　population　density　and　fixed　assets　density.　The　inside　state　sets　of　urban　earthquake　disaster　prevention　system　are　determined　as　disaster　resistant　ability　of　buildings,　disaster　resistant　ability　of　lifelines　and　investment　dynamics　in　disaster　prevention　per　urban　built-up　area.　The　system　output　is　urban　seismic　risk.　The　calculative　results　show　the　model　presented　in　this　study　can　analyze　the　influence　of　system　periphery　intensity　and　inside　state　on　seismic　risk　and　can　control　urban　seismic　risk　by　adjusting　the　parameters　of　system　periphery,　the　system　inside　state　and　human　influence　intensity.