Based　on　coupled-mode　theory　and　mechanics　of　materials,　the　four-core　fiber　Bragg　gratings　(F-CFBG)　transverse　force　sensing　rules　according　to　transverse　load,　direction　and　fiber　outer　diameter　were　studied.　The　F-CFBG　reflection　spectrum　split　into　two　main　peaks　under　transverse　load.　The　resonant　wavelength　changes　and　the　space　between　those　two　peaks　are　proportional　with　the　applied　transverse　force　magnitude,　but　decrease　as　the　transverse　orientation　increases.　The　ratio　of　the　peak　splitting　between　two　cores　at　90°　separation　only　lie　on　the　orientation　of　transverse.　The　numerical　analysis　of　F-CFBG　wavelength　shift　with　different　outer　diameters　was　also　presented　respectively.　On　condition　that　normalized　frequency　keeps　constant,　the　wavelength　change　and　the　space　between　those　two　peaks　increase　as　the　fiber　outer　diameter　decreases,　but　the　increase　degree　lie　on　the　transverse　orientation.