To study the deformation performance of I-section beam with longitudinally variable thickness flanges, 2 welded I-section variable thickness steel beams and 2 normal beams under static loading were conducted, and corresponding numerical simulation was performed using the ANSYS. The special material of the flanges was longitudinally profiled steel plates of Q345GJC, and single point loading and two point loading were adopted respectively. All of the four specimens occurred local buckling when the lateral displacement was constrained. The results show that the beam could not only ensure the bearing capacity with less steel but also safety margin, and also can meet the deformation requirements of compact cross section, compared with the beam with uniform thickness. The finite element results are in good agreement with the test results. The test results can be used for the plastic design of I-section beam with longitudinally variable thickness flanges.