Fractional　frequency　offshore　wind　power　system　(FFOWPS)　is　a　new　type　of　offshore　wind　power　system,　which　is　proved　to　be　more　economical　and　has　technological　advantages　for　medium　and　long　transmission　lines.　Modular　multilevel　matrix　converter　(M3C)　is　the　most　expected　AC/AC　converter　that　may　be　used　in　FFOWPS.　However,　there　is　no　research　published　about　the　protection　of　M3C-based　FFOWPS.　In　this　paper,　the　challenges　for　the　protection　of　M3C-based　FFOWPS　have　been　analyzed.　They　can　be　summarized　as　follows,　(i)　M3C　and　wind　turbines　are　all　electronic　devices.　Their　intrinsic　characteristics　and　control　strategy　will　significantly　complicate　the　fault　characteristics,　(ii)　The　decrease　in　system　frequency　will　extend　the　time　of　phasor-based　protection　methods.　To　solve　the　above　problems,　the　idea　of　model　recognition　is　introduced　in　fault　identification　for　FFOWPS　based　on　M3C,　which　discriminates　faults　by　model　differences　instead　of　comparison　of　voltage　or　current.　Then　a　novel　time-domain　pilot　protection　is　proposed　for　FFOWPS　based　on　M3C.　An　M3C-based　FFOWPS　is　built　in　MATLAB/Simulink　and　the　simulations　have　verified　the　adaptability　of　the　proposed　method.　©　2022　IEEE.