The　low-temperature　brittle　failure　of　materials　could　aggravate　the　brittle　characteristics　of　shear　failure　of　RC　beams,　which　urgently　needs　scientific　research.　This　paper　aims　to　investigate　the　shear　behavior　and　corresponding　size　effect　of　reinforced　concrete　(RC)　beams　at　low　temperature　by　numerical　analysis.　Firstly,　a　twostage　thermo-mechanical　coupled　mesoscale　simulation　method　was　developed,　which　considered　the　mesostructure　characteristics　of　concrete　as　well　as　the　ice-strengthening　effect.　Based　on　the　mesoscale　simulation　method　validated　through　a　series　of　tests,　the　progressive　shear　failure　process　of　RC　beams　at　room　and　low　temperatures　was　captured.　The　influences　of　temperature　(T　=　20,　-30,　-60　and　-90　degrees　C),　structural　size　(crosssectional　height　H　=　300,　600,　and　1200　mm)　and　shear　span-to-depth　ratio　(a/h0　=　1.0,　1.6,　and　2.3)　on　key　indexes　of　shear　behavior　were　quantitatively　analysed.　The　results　indicate　that　compared　to　that　at　room　temperature,　the　shear　failure　process　of　RC　beams　at　low　temperature　is　more　rapid,　exhibiting　more　obvious　brittle　characteristic.　When　the　temperature　drops　from　20　degrees　C　to　-90　degrees　C,　the　nominal　shear　strength　of　RC　beams　is　enhanced　with　a　maximum　increase　of　57.0　%　(for　H　=　300　mm)　while　the　mid-span　displacement　is　reduced　with　a　maximum　decrease　of　46.8　%　(for　H　=　300　mm).　Compared　to　that　at　room　temperature,　the　size　effect　on　nominal　shear　strength　of　RC　beams　at　low　temperature　is　enhanced,　with　a　maximum　increase　of　20.4　%.　Finally,　considering　the　influences　of　low　temperature　and　shear　span-to-depth　ratio　on　the　nominal　shear　strength,　an　improved　size　effect　analytical　model　for　was　established,　which　can　predict　the　shear　capacity　of　different　sized　RC　beams　at　low　temperatures.　This　paper　can　provide　a　reference　for　the　safe　application　of　reinforced　concrete　structures　in　low　temperature　environments,　and　lay　a　foundation　for　the　formulation　of　reinforced　concrete　structure　design　specifications　at　low　temperature.