A　numerical　model　utilizing　3D　mesoscale　simulation　methods　was　developed　to　investigate　the　influence　of　strain　rate　on　the　torsional　performance　of　geometrically　similar　Basalt　Fiber　Reinforced　Polymer　bars-reinforced　concrete　(BFRP-RC)　beams,　as　well　as　the　corresponding　size　effects.　The　model　incorporates　concrete　heterogeneity,　material　strain　rate　effects,　and　the　dynamic　bond-slip　relationship　between　BFRP　bars　and　concrete.　The　torsional　performance　of　BFRP-RC　beams　with　different　structural　sizes　and　stirrup　ratios　was　analyzed　under　different　strain　rates.　The　study　yielded　the　following　findings:　(1)　The　damage　degree　of　BFRP-RC　beams　increases　with　the　rising　strain　rate.　(2)　Increasing　strain　rate　and　stirrup　ratio　enhances　the　beams’　torsional　strength　and　ductility　while　attenuating　the　size　effect,　albeit　not　eliminating　it.　(3)　The　impact　of　increasing　strain　rate　on　beam　strength,　ductility,　and　size　effect　outweighs　that　of　increasing　stirrup　ratio.　Finally,　based　on　the　Bažant　size　effect　law　(SEL)　combined　with　the　simulation　results,　a　new　size　effect　law　was　proposed　that　can　quantitatively　consider　the　effect　of　strain　rate　and　stirrup　ratio　on　the　torsional　strength　of　BFRP-RC　beams.　©　The　Author(s)　2024.