The invention relates to an all-solid-state deep ultraviolet laser, in particular to a monocrystalline-diamond continuous wave tunable deep ultraviolet laser. The monocrystalline-diamond continuous wave tunable deep ultraviolet laser is provided with a 456nm single-frequency blue-light laser, a transverse mode matching lens, a first laser resonator mirror, a second laser resonator mirror, a third laser resonator mirror, a fourth laser resonator mirror, a laser gain medium, a frequency doubling crystal, a photodiode and a PDH (Pound-Drever-Hall) controller. The 456nm single-frequency blue-light laser, the transverse mode matching lens, the first laser resonator mirror, the frequency doubling crystal and the second laser resonator mirror are arranged on a first optical axis sequentially from left to right, and the third laser resonator mirror, the laser gain medium and the fourth laser resonator mirror are arranged on a second optical axis sequentially from left to right; the photodiode is located on the left rear side of the first laser resonator mirror, and the PDH controller is connected with the photodiode and the fourth laser resonator mirror; the first optical axis is parallel to the second optical axis. The all-solid-state deep ultraviolet laser is small in size, high in efficiency, long in service life, good in beam quality, easy for system integration and easily practical.