An upper frame structure for supporting a cab of construction machinery is disclosed, which can support the cap mounted on the upper frame when a vertical load, which is so much that the cab structure is plastically deformed, is applied to the upper frame. The upper frame structure includes a center frame having a bottom plate on which a swing ring gear is mounted and a pair of side plates vertically fixed to the bottom plate and on which operation devices are mounted; a left frame mounted on a left side of the center frame and having a left side frame on which the cab is mounted; reinforcement members each of which has one end fixed by welding to a side surface of the side plate of the center frame and the other end fixed by welding to a side surface of the left side frame, and on which the cap is mounted; and a fastening member installed on the bottom plate to offset a load vertically applied to the cab and a load laterally applied to the cab, and fixing the reinforcement member to the bottom plate.