Traditional　way　to　enable　frequency　support　provided　by　MTDC　system　for　the　connected　weak　AC　systems　is　to　modify　the　power　reference　of　each　converter　under　standard　power-sharing　droop　control　based　on　the　weak　grid＇s　frequency　deviations.　However,　this　scheme　artificially　couples　all　asynchronous　AC　systems　to　tackle　with　the　frequency　disturbances　of　any　connected　AC　systems,　which　might　result　in　broader　frequency　disturbance　propagations.　To　overcome　this,　this　study　proposes　a　decoupled　frequency　control　(DFC)　scheme　to　improve　weak　AC　systems　frequency　stabilities.　First,　the　relationship　between　the　active　power　and　power　set　points　of　each　converter　is　derived　by　the　linearization　of　MTDC　power　flow　equations.　Then,　the　frequency　coupling　properties　of　each　AC　system　with　traditional　frequency　control　are　derived,　and　it　shows　that　the　frequency　disturbance　will　be　propagated　to　all　the　connected　AC　systems　with　traditional　control.　Furthermore,　the　control　law　of　each　converter　in　the　proposed　DFC　is　developed　through　the　defined　decoupled　matrix,　and　the　independent　frequency　regulation　characteristic　of　each　converter　can　be　obtained.　Finally,　numerical　simulations　have　demonstrated　the　effectiveness　of　the　proposed　DFC　scheme　in　events　of　load　changes　and　converter　loss　in　the　main　and　weak　AC　grid.　©　2020　Institution　of　Engineering　and　Technology.　All　rights　reserved.