In　this　study,　a　3D　meso-scale　numerical　simulation　method　is　established　for　analyzing　the　shear　failure　of　carbon　fiber　reinforced　polymer　(CFRP)　strengthened　RC　beams　without　stirrups.　The　inhomogeneity　of　concrete,　the　bond-slip　relationship　between　steel　bars　and　concrete,　and　the　interaction　between　CFRP　and　concrete　are　considered.　Based　on　this　method,　effects　of　shear-span　ratio　and　CFRP　fiber　ratio　on　the　shear　performance　and　nominal　shear　strength　size　effect　of　CFRP　strengthened　RC　beams　are　investigated.　Results　indicate　that　the　size　effect　of　CFRP　strengthened　RC　beams　is　obvious,　however,　the　shear-span　ratio　has　little　impact　on　the　size　effect　of　nominal　shear　strength　of　RC　beams.　The　CFRP　fiber　ratio　enhances　the　shear　strength　of　RC　beams　on　the　one　hand,　while　weakens　the　shear　size　effect　of　RC　beams　on　the　other　hand.　However,　its　influence　on　the　shear　performance　of　RC　beams　remains　basically　unchanged　with　varying　shear-span　ratios,　indicating　that　there　is　no　coupling　effect　between　the　shear-span　ratio　and　CFRP　fiber　ratio.　The　shear-span　ratio　significantly　influences　both　the　shear　bearing　capacity　of　RC　beams　and　the　shear　contribution　of　CFRP　sheets,　with　the　increase　in　the　shear-span　ratio,　the　shear　bearing　capacity　of　RC　beams　decreases　to　varying　degrees,　and　the　shear　contribution　of　CFRP　sheets　increases　initially　and　then　decreases.　Finally,　the　shear　strength　size　effect　law　(SEL)　of　CFRP　strengthened　RC　beams　is　proposed,　which　can　quantitatively　describe　the　effects　of　the　shear-span　ratio　and　the　CFRP　fiber　ratio.　©　2024　Elsevier　Ltd