The invention discloses a cleaning system and a combined grain harvester adopting the cleaning system. A cyclone separation cylinder of the cleaning system comprises a cylinder body; the middle section of the cylinder body is a cylindrical section, and the upper and lower sections are cone sections; the upper end of the cylinder body is communicated with a gettering fan, and the cylindrical section is communicated with a winnower through a separation pipe; the side wall of the separation pipe, which is far from the axis of the cylinder body, is raised from the wall of the cylindrical section of the cylinder body; and ventilation holes are distributed at intervals on the side wall of the lower cone section of the cylinder body. The cleaning system with the structure ensures that the materials to be cleaned is far from the axis of the cylinder body as much as possible when entering the cyclone separation cylinder so that the cleaning loss can be reduced without increasing the diameter of the cylinder body. In addition, the air entering the lower cone section of the cylinder body from the ventilation holes forms an air cushion layer on the inner wall of the lower cone section so that part of the light impurities entered the lower cone section suspends under the action of the air cushion layer, is sucked out by the gattering fan and cannot block the lower cone section hereby.