Abstract: By means of macro examination,chemical composition analysis,mechanical properties test,metallographic examination and hardness test,the bending property unqualified reason of 36U support profile steel used in mine tunnel was analyzed. The results showed that the main reason was that the content of C and Si exceeded standard,the content of Mn exceeded standard consumedly,large amounts of bainite and less ferrite were produced,and the hardness was a times more than the normal hardness,led to the property of 36U profile steel was abnormal and the brittleness fracture happened in bending process.