A lettering apparatus and a lettering method for a beer barrel are provided. The lettering apparatus includes a worktable, a hydraulic push device installed on the worktable, a first lettering device, and a second lettering device. The second lettering device includes a rotatory disk seat with the Z-axis as its axis. The rotatory disk seat is adapted to install a cask thereon. A circumferential surface of the rotatory disk seat is provided with an inner mold for lettering on an inner side of the cask. The first lettering device includes a connecting frame. The connecting frame is movable linearly along the Y-axis. The connecting frame is provided with an outer mold for lettering on an outer side of the cask. The hydraulic push device controls the outer mold of the connecting frame to move linearly along the X-axis for the outer mold and the inner mold to letter on a side wall of the cask.