A grinding device, comprising a body configured as a hollow barrel having an opening and a first fixing element positioned close to the opening; an inner cap configured to mount to an inner wall of an end close to the opening of the body having a second fixing element corresponding to the first fixing element; wherein an aperture is provided on the inner cap, and a first leak-proof seal is configured to surround the aperture; an outer cover having a second leak-proof seal, wherein the second leak-proof seal engages the first leak-proof seal for leak-proofing; and a grinding element positioned inside the body and capable of moving therein freely; wherein an area of any cross section of the grinding element is bigger than the aperture of the inner cap.