In　order　to　reflect　and　reveal　the　shear　failure　process　and　mechanism　of　carbon　fiber　reinforced　plastic　(CFRP)　sheets　strengthened　reinforced　concrete　(RC)　beam,　a　mechanical　analysis　model　is　established　by　using　a　3D　meso-scale　simulation　method.　The　model　considers　the　heterogeneity　of　concrete　materials,　the　interaction　behaviors　between　reinforcements　and　concrete,　and　the　complex　interaction　behaviors　between　CFRP　sheets　and　concrete,　simultaneously.　Different　types　of　meso-scale　models　of　RC　beam　strengthened　with　CFRP　sheets　are　designed　to　explore　the　influences　of　stirrup　ratio　and　CFRP　fiber　ratio　on　the　nominal　shear　strength　and　size　effect　behaviors.　Test　results　indicate　that:　1)　The　action　mechanisms　of　stirrups　and　CFRP　sheets　are　similar;　2)　The　nominal　shear　strength　of　CFRP　sheets　strengthened　RC　beams　increases　with　the　increase　of　stirrup　ratio　and　CFRP　fiber　ratio,　while　the　degree　of　the　increasing　range　decreases;　3)　The　nominal　shear　strength　of　RC　beams　strengthened　with　CFRP　sheets　decreases　with　the　increase　of　section　size,　nevertheless　increasing　the　stirrup　ratio　and　the　CFRP　fiber　ratio　will　weaken　the　size　effect;　4)　Based　on　the　numerical　test　results,　a　novel　size　effect　law　simultaneously　considering　the　stirrup　ratio,　the　CFRP　fiber　ratio　and　the　mutual　effects　between　them　is　proposed.