43 longitudinal fillet welded connection specimens were first heated to specified temperature levels, and then cooled to ambient temperature and stretched to failure. Seven post fire temperatures ranging from 200℃ to 900℃ were considered, and two cooling methods including natural cooling and water cooling were adopted in the test. The tri linear model was proposed to calculate the variation factor of ultimate shear strength of longitudinal fillet weld after exposure to elevated temperature. Results show that the fracture angles of longitudinal fillet welds after exposure to elevated temperature are about 45°, indicating that fracture of these welds is caused by shear stress. When the post fire temperature is above 400℃, the ultimate shear strength of longitudinal fillet weld starts to decrease significantly, and the decrease range reaches about 25% at 700℃.The ultimate shear strength of longitudinal fillet weld starts to increase when the post fire temperature is above 700℃, and the cooling methods have a significant effect on the strength variation degree at this stage. The predicted values of variation factors of ultimate shear strengths of longitudinal fillet welds after exposure to elevated temperature calculated by tri linear model agree well with the test values.