Based　on　a　practical　long-distance　large-capacity　transmission　engineering　case,　the　small-signal　stability　of　fractional　frequency　transmission　system　(FFTS)　with　Y-connected　modular　multi-level　converter　(Y-MMC)　is　mainly　researched　in　this　paper.　Different　from　the　previous　control　strategies　of　Y-MMC,　this　paper　establishes　a　frequency　decoupling　mathematical　model　and　proposes　the　decoupling　control　strategies.　Then,　the　small-signal　model　of　the　Y-MMC　system　is　obtained.　Considering　the　existence　of　phase　locked　loop　(PLL)　will　deteriorate　the　stability　of　the　system　under　weak　grid,　this　paper　applies　the　short-circuit　ratio　(SCR)　for　analysis　and　proposed　a　new　structure　of　PLL,　called　phase-shifted　PLL　(PS-PLL),　to　improve　the　small-signal　stability　of　the　system.　In　addition,　for　better　stability　and　dynamic　response　performance　of　the　closed-loop　control　system,　this　paper　proposes　a　new　parameter　optimization　objective　function　for　parameters　adjustment.　Controller　parameters　optimization　is　conducted　by　using　the　particle　swarm　optimization　(PSO)　algorithm　and　stability　evaluation　of　the　system　is　conducted　based　on　the　eigenvalue　distribution.　Finally,　the　feasibility　and　superiority　of　the　proposed　strategies　and　optimization　are　verified　in　MATLAB/　Simulink.　©　2022　The　Authors.　IET　Generation,　Transmission　&　Distribution　published　by　John　Wiley　&　Sons　Ltd　on　behalf　of　The　Institution　of　Engineering　and　Technology.