Abstract: To deal with the problem of relatively low level specified, both economically and scientifically, in current fire protection and prevention regulations, its global mechanical properties of the portal frame steel structure were studied analytically in the fire disaster, models with typical spans and heights were established for a series of portal frame structures and their mechanical properties were analyzed numerically during the 1 h burning process in the fire disaster, from which the failure modes and the rules of stress and deformation of the portal frame steel structure were obtained. With a real fire disaster example, influences of the surrounded members on the mechanical property in the fire disaster were discussed.