By using ISO9705 Room/Corner Fire Test Rig, thin steel tubes were tested. The response behaviors of steel element under thermal-mechanical coupling effect in a simulated corner fire were studied. In the experiment, the heat release rate and temperature field of the fire, temperature and deflection of the steel element and the temperature of the gas close to steel surface were measured. Effects of the temperature elevation and distribution of the steel element under corner fire conditions along with the fire growth on the deflection were analyzed. Results show that there exists significant temperature difference along the longitude of the steel and an unsymmetrical deflection variation. These indicate that assumptions of the uniform temperature distribution along the longitude direction in previous studies were improper. Under this non-uniform temperature distribution, failure resulted from the local yield of the higher temperature zone of the element is more inclined to occur. Indeed, this non-uniform temperature distribution caused by the corner fire is more dangerous to the fire safety of steel elements than the even case under same fire size.