The　ductile-to-brittle-transition　(DBT)　of　Sn　occurring　when　the　temperature　decreases　to　a　critical　temperature　poses　a　restriction　on　the　Sn-based　solders　in　the　cryogenic　electronics　interconnection.　Previous　investigations　about　the　DBT　mainly　adopt　the　Charpy　impact　test,　however,　the　transient　property　of　Charpy　impact　test　makes　it　difficult　to　capture　the　deformation　features　accompanying　DBT　and　clearly　elucidate　the　DBT　mechanism.　In　this　paper,　we　studied　the　plastic　deformation　of　Sn　before　cryogenic　brittle　fracture　through　in-situ　cryogenic　uniaxial　tensile　experiments　under　optical　microscopy.　It　is　found　that　{301}　deformation　twins　were　activated　and　thickened　before　the　brittle　fracture.　Due　to　the　occurrence　of　deformation　twins,　multiple　twin-grain　boundary　intersections　and　twin-twin　intersections　formed　within　Sn　grains,　which　could　strongly　prevent　dislocation　slip　and　thus　lead　to　stress　concentration.　As　a　result,　the　brittle　cracks　initiate　and　propagate　along　these　intersections.　According　to　the　suggested　specific　mechanism,　we　designed　a　green　pre-treatment　method　of　pre-twining-RT　tempering　to　increase　the　cryogenic　ductility　of　Sn-based　solder　alloys　without　any　energy　consumption.　Eventually,　the　fracture　elongation　of　Sn　at　LNT　was　significantly　improved　from　~6%　to　~14%.　　©　2022　IEEE.