A bone drilling cover device (A) comprises an upper fastening member (30) and a lower fastening member (50). The upper fastening member (30) includes a first base portion (31) and a first coupling portion (32). The lower fastening member (50) includes a second base portion (51) and a second coupling portion (52) engagable with the first coupling portion (32). The second base portion (51) is composed of at least one first arm (511) and at least one second arm (512). The first arm (511) has a length (X) greater than a length (Y) of the second arm (512). A total length (X+Y) of the first arm (511) and the second arm (512) is greater than a diameter (D) of a drilled hole (73). Through the first arm (511) and the second arm (512) having different lengths, the first arm (511) and the second arm (512) can be inserted in an oblique manner and positioned to the underside of the drilled hole (73) for covering the drilled hole (73) of any bone. The operation is quick and easy. The diameter of the drilled hole (73) is minimized for reducing damage to the bone structure. When the invention is used for retaining a cranial bone (72), the first arm (511) and the second arm (512) are configured to span and hold the cranial bone (72) and a cranium (70). The invention can be widely used.
