The invention provides a centralized transmission reducer for rolling hot-rolled seamless steel tubes and a reducing process of hot-rolled seamless steel tubes by using the reducer. The reducing process comprises the following steps of: determining the ratio of the rotating speed of a superimposed motor to the rotating speed of a main motor by utilizing the ratio of the outer diameter to the wall thickness of the hot-rolled seamless steel tube; adjusting the rotating speeds of the superimposed motor and the main motor according to the ratio of the rotating speeds; utilizing the torque rotatably output by the main motor and the superimposed motor at respective rotating speeds, so that a gear set inputs sequentially increasing rotating speeds to multiple passes; and utilizing multiple passes to reduce the diameter of the hot-rolled seamless steel tube, wherein the ratio of the rotating speed of the superimposed motor to the rotating speed of the main motor decreases with the increase of the ratio of the outer diameter to the wall thickness of the hot-rolled seamless steel tube. By applying the centralized transmission reducer provided by the invention in the production process of the steel tubes, the wall thickness uniformity of the steel tube is improved.