A ceiling lamp consists of a lamp panel (1), a lamp cover (2) locked on the lamp panel (1), and a lamp body (3) mounted in the lamp panel (1). The lamp panel (1) consists of a panel body (11) provided with a panel chamber, a step slot (12) disposed on the inner wall of the panel body (11) from the top to the bottom in a diameter-reduced manner, water-proof rings (13) respectively disposed on turns of the step slot (12), an inwards-recessed groove (14) located on the inner wall of the opening end, and a tightening screw (15) penetrating from the outside to the inside of the panel body (11). The lamp body (3) consists of a flexible strip (32), a diffusion plate (311), a light guide plate (312), and light reflection paper (313). The lamp cover (2) has two structures. One structure has a mounting hole reserved on a top wall, and consists of a bottom plate (21), a Z-shaped support (23) and a circlip reed (24). The other structure is mounted by means of screws (25), and consists of a bottom plate (21), a circlip (28) and a hanging plate (29). Two different mounting methods are formed based on the two lamp cover structures. The ceiling lamp has a thickness of smaller than or equal to 15 millimeters, has good waterproof performance, is suitable for an external power supply ranging from 10 voltages to 30 voltages, and can satisfy use requirements on a yacht, a recreational vehicle and other special environments.