The invention relates to a combined gear box driving a drying cylinder of a paper machine. The combined gear box is formed by assembling a corner cabinet module, a connecting flange and a reduction box module sequentially, wherein a cabinet body of the corner cabinet module is internally provided with an input shaft and a driving shaft with the two ends extending out of the cabinet body, the input shaft and the driving shaft are in quadrature, and a bevel gear pair with the transmission ratio being 1:1 is arranged between the input shaft and the driving shaft. An output shaft and a reduction gear shaft which are parallel to each other are arrayed on the vertical section in a box body of the reduction box module from top to bottom. The two ends of the output shaft provided with a reduction big gear penetrate through the box body of the reduction box module. A reduction gear pair is arranged between the reduction big gear and the reduction gear shaft. A transition shaft and the reduction gear shaft which are in quadrature are located on the same horizontal section. A reduction bevel gear pair is arranged between the transition shaft and the reduction gear shaft. The two ends of the transition shaft extend out of the box body of the reduction box module. The outer walls, at the two ends of the driving shaft, of the cabinet body of the cornet cabinet module and the outer walls, at the two ends of the transition shaft, of the box body of the reduction box module are all provided with connecting bolts corresponding to connecting holes in the connecting flange. The cabinet body of the cornet cabinet module and the box body of the reduction box module are fixed to a whole through the connecting flange after the driving shaft and the transition shaft are connected in a butt joint mode.