Rotor　angle　stability　of　meshed　AC/DC　system　due　to　the　decreased　effective　inertia　with　the　high　penetration　of　renewables　has　become　a　major　challenge.　An　easy　and　feasible　way　to　enhance　the　rotor　stability　of　power　system　provided　by　Multi-terminal　DC　(MTDC)　grid　is　to　fast　modulate　the　power　reference　of　standard　power　sharing　droop　control　of　each　converter　based　on　the　rotor　speed　deviations　of　the　selected　generators.　However,　this　feedback　based　scheme　does　not　consider　the　coordination　of　each　converter,　which　cannot　render　an　optimal　solution　for　the　stability　issue　of　the　system　and　cannot　make　full　use　of　the　connected　converter　capability.　To　overcome　this,　this　paper　proposes　an　optimal　based　control　of　MTDC　grid　for　improving　the　rotor　angle　stability　of　power　system.　First,　MTDC　grid　model　is　well　established　by　the　linearization　of　power　flow　equations　of　DC　grid.　Then　AC/DC　meshed　system　model　with　the　power　modulation　control　of　DC　grid　is　further　established.　Furthermore,　the　optimal　based　control　of　MTDC　grid　is　deduced　by　the　online　minimization　of　the　rotor　angle　deviations　of　generators　and　the　control　energy.　Finally,　the　nonlinear　numerical　simulations　of　one　simple　three　generators　nine　buses　AC/DC　meshed　system　have　demonstrated　the　effectiveness　of　the　proposed　optimal　based　scheme　in　three　phase　fault　and　sudden　load　changes　in　AC　system.　©　2019　IEEE.