To　improve　the　pedestrian　outflow　in　panic　situations　by　suitably　placing　an　obstacle　in　front　of　the　exit,　it　is　vital　to　understand　the　physical　mechanism　behind　the　evacuation　efficiency　enhancement.　In　this　paper,　a　robust　differential　evolution　is　firstly　employed　to　optimize　the　geometrical　parameters　of　different　shaped　obstacles　in　order　to　achieve　an　optimal　evacuation　efficiency.　Moreover,　it　is　found　that　all　the　geometrical　parameters　of　obstacles　could　markedly　influence　the　evacuation　efficiency　of　pedestrians,　and　the　best　way　for　achieving　an　optimal　pedestrian　outflow　is　to　slightly　shift　the　obstacle　from　the　center　of　the　exit　which　is　consistent　with　findings　of　extant　literature.　Most　importantly,　by　analyzing　the　profiles　of　density,　velocity　and　specific　flow,　as　well　as　the　spatial　distribution　of　crowd　pressure,　we　have　proven　that　placing　an　obstacle　in　panic　situations　does　not　reduce　or　absorb　the　pressure　in　the　region　of　exit,　on　the　contrary,　promotes　the　pressure　to　a　much　higher　level,　hence　the　physical　mechanism　behind　the　evacuation　efficiency　enhancement　is　not　a　pressure　decrease　in　the　region　of　exit,　but　a　significant　reduction　of　high　density　region　by　effective　separation　in　space　which　finally　causes　the　increasing　of　escape　speed　and　evacuation　outflow.　Finally,　it　is　clearly　demonstrated　that　the　panel-like　obstacle　is　considerably　more　robust　and　stable　than　the　pillar-like　obstacle　to　guarantee　the　enhancement　of　evacuation　efficiency　under　different　initial　pedestrian　distributions,　different　initial　crowd　densities　as　well　as　different　desired　velocities.　©　2016　Elsevier　B.V.