In experimental investigation on mechanical properties of Q235 steel after high temperature treatment,surface characteristic and influences of the important parameters including heating temperature and temperature exposure time on the mechanical properties of steel after high temperature treatment were studied.Formulas for Possion's ratio-stress ratio,tensile stressstrain,yield strength-heating temperature,tensile strength-heating temperature of steel after high temperature treatment are proposed.The result show that: with the heating temperature rising,yield strength and tensile strength tend to decrease as a whole,while elastic modulus of the steel and Possion's ratio almost remain unchanged;with increasing temperature exposure time,the mechanical properties of the steel after high temperature treatment are hardly influenced; the calculated results by above-mentioned formulas agree well with test ones.