A grinding-disc device for power tools has an outer cover, a grinding tray and a grinding element. The outer cover has a casing and a mounting tube. The casing has a chamber, a connecting tube, at least one engaging hole and at least one engaging arm. The mounting tube is connected to the connecting tube. The grinding tray is detachably connected to the outer cover and has a body and at least one engaging element. The body is mounted below the casing and has a tube seat and a connecting board. The at least one engaging element is formed on the body, engages the at least one engaging arm to hold the grinding tray with the outer cover, and each one of the at least one engaging element has an engaging hook. The grinding element is detachably connected to the grinding tray below the outer cover.