Arm　reactor　is　an　important　part　of　the　modular　multilevel　converter　(MMC)　and　its　parameter　designing　is　critical　for　stable　operation　of　the　converter.　Beginning　with　the　analysis　of　harmonic　current,　by　analyzing　the　structure　and　working　principle　of　the　converter,　the　constraint　relationship　between　circulating　current　amplitude　and　arm　reactance　was　deduced　in　detail,　and　then　the　system　general　matrix　equation　was　obtained.　Based　on　the　above　analysis,　this　paper　proposes　an　arm　reactor　parameter　designing　method　which　can　avoid　system　resonance,　meet　the　requirements　of　system　power　transmission　and　circulating　current.　Take　Zhoushan　flexible　HVDC　project　for　example,　the　simulation　results　based　on　Matlab/Simlink　verify　the　proposed　design　method　is　feasible.　©,　2015,　Power　System　Technology　Press.　All　right　reserved.