A paper shredder with an allowable thickness warning function is provided. The paper shredder includes a casing and an upper cover. An input tray is disposed on the casing for placing plural papers thereon. A pressing member and a thickness detecting device are installed on the upper cover. When the pressing member is contacted with the plural papers on the input tray and pushed by the plural papers, the pressing member is moved. The thickness detecting device is located near the pressing member. Moreover, in response to the movement of the pressing member, the thickness detecting device may be triggered. If an overall thickness of the plural papers exceeds an allowable thickness, the pressing member is pushed by the plural papers. Consequently, the thickness detecting device is triggered by the pressing member to generate a warning signal.