The invention provides a preheating device for groove welding of a large-wall-thickness pipe fitting and belongs to the field of casting technologies. The preheating device comprises a preheating mainbody device, a fixing device and an air inlet adjusting device. The preheating main body device is fixed to the inner side of the pipe fitting through the fixing device and located on the back side of a groove. By adjusting the fixing device, the preheating main body device and the pipe fitting are concentrically fixed, and an interval is reserved between the preheating main body device and the inner wall of the pipe fitting, so that the groove of the pipe fitting can be uniformly preheated through the preheating main body device. One end of the fixing device is arranged on the preheating main body device, and the other end of the fixing device is arranged on the inner wall of the pipe fitting, so that the pipe fitting and the preheating main body device are relatively fixed. Fire spray openings are uniformly formed in the side wall of the preheating main body device so that it can be guaranteed that the groove of the pipe fitting is fully preheated, the groove of the pipe fitting isheated by using an open fire manner, the groove can be fully heated by flames according to the radian and depth of the groove, and the flame temperature is easy to control.