Through　a　3D　meso-scale　numerical　simulation　method,　the　shear　failure　mechanical　analysis　model　for　large-sized　GFRP　reinforced　concrete　(GFRP-RC)　beams　was　established.　The　influences　of　parameters　of　major　importance　on　the　crack　angle　and　on　P-Δ　curve　of　GFRP-RC　beams　was　discussed　through　this　model,　such　as　shear　span　ratio,　longitudinal　reinforcement　ratio,　and　beam　depth.　Based　on　the　modified　compression　field　theory　(MCFT),　a　calculation　model　for　the　average　crack　width　was　established,　considering　the　variation　of　crack　width　along　the　beam　depth,　which　improves　the　calculation　accuracy　of　the　simplified　formula　for　MCFT.　A　design　method　was　proposed　for　shear　bearing　capacity　of　GFRP-RC　beams　without　stirrups　considering　the　influences　of　the　shear　span　ratio　and　size　effect　on　the　crack　angle　and　on　shear　capacity.　The　model　proposed　is　validated　by　comparing　with　the　corresponding　experimental　measurements　of　213　GFRP-RC　beams,　and　with　the　consideration　of　the　average　crack　width　and　size　effects,　utilizing　the　model　proposed　will　garner　the　satisfactory　results　with　remarkable　higher　accuracy　compared　to　current　used　design　codes.　Therefore,　the　improved　MCFT　model　proposed　provides　a　theoretical　reference　for　the　shear　design　of　GFRP-RC　beams.　©　2023　Tsinghua　University.　All　rights　reserved.