A reinforced upper frame for supporting a cabin of heavy construction equipment using a welding deformation preventing structure and a method of manufacturing the same are disclosed, in which even ifa side frame is welded to a machined center frame, there is no thermal deformation around the machined portion. The reinforced upper frame includes a center frame having a lower plate machined to accommodate a swing ring gear, a lateral plat machined to accommodate an operation device, and a welding deformation preventing structure fixed to one side of the lower plate and lateral plate prior to welding, in which a part of the cabin is mounted on the welding deformation preventing structure, and left and right frames each having a side channel disposed in a longitudinal direction of the centerframe and a side frame welded to the structure for connecting the side channel and the center frame.