Abstract: This study investigated the buckling behavior of I-section beams with a floor slab or profiled steel sheeting attached to the top flange. The upper and lower flanges were modeled as individual thin-walled members， and the web as a plate subjected to axial forces and bending moment. A total potential energy expression was established accordingly. Displacement functions were formulated on the premise that the web remains perpendicular to the flanges， and these functions were substituted into the total potential energy expression to obtain the critical load. The torsional buckling of the section around the upper flange was analyzed based on thin-walled member theory. Considering the torsional restraint on the upper flange， the formula for critical stress was derived. When the floor slab was sufficiently thick， the section exhibited distortional buckling， and the corresponding critical distortional buckling stress formula was proposed. By comparing the above three groups of solutions， a simple formula with satisfactory accuracy and a reasonable safety margin was developed by modifying the thin-walled member theory formula for fixed-axis torsional buckling， which is applicable to the calculation of distortional buckling stress when the upper flange is permitted to rotate.