The present disclosure provides a top cover assembly for a battery, a battery and a device using a battery as a power source. The top cover assembly includes: a cover plate body having a liquid injection hole; a mounting part configured to be connected with the cover plate body and arranged around the liquid injection hole; a sealing element configured to be connected with the mounting part, wherein the sealing element includes a first top wall and a first side wall connected to the periphery of the first top wall, the first top wall is configured to cover the liquid injection hole, and the first side wall is configured to be in press fit with the mounting part to achieve sealing of the liquid injection hole; and a fixing element configured to be connected with the cover plate body and to fix the sealing element to the mounting part.