The　purpose　of　the　present　study　is　to　explore　the　influence　of　fiber　ratio　on　the　shear　failure　and　size　effect　of　reinforced　concrete　(RC)　beams　strengthened　in　shear　with　externally　bonded　CFRP　sheets.　A　three-dimensional　meso-scale　numerical　model　that　can　consider　concrete　heterogeneities　and　CFRP-concrete　interactions　was　established.　The　influence　of　fiber　ratio　on　size　effect　in　shear　failure　was　studied.　Moreover,　a　size　effect　law　is　established　and　verified　to　predict　the　nominal　shear　strength　of　CFRP-strengthened　concrete　beams,　in　which　the　contribution　of　the　CFRP　fiber　ratio　is　quantitatively　considered.　The　simulation　results　indicate　that,　1)　The　cross-section　size　has　a　great　influence　on　the　shear　strength　of　CFRP-strengthened　RC　cantilever　beams,　and　CFRP　has　a　greater　contribution　to　the　shear　capacity　of　small-sized　beams;　2)　The　increase　of　fiber　ratio　strengthen　the　shear　capacity　of　CFRP-strengthened　RC　cantilever　beam　and　weakens　the　size　effect　on　shear　strength;　3)　The　existence　of　CFRP　cannot　increase　the　shear　strength　of　RC　beams　indefinitely,　and　the　increase　of　shear　strength　is　obvious　under　the　condition　of　small　fiber　ratio;　4)　The　proposed　size　effect　law　for　shear　failure　of　CFRP-strengthened　RC　beams　can　well　describe　the　quantitative　influence　of　fiber　ratio,　and　it　is　consistent　with　the　available　test　data.