The present invention provides an ultra violet irradiating device for aligning liquid crystal and also an water-cooling coaxial tube. The ultraviolet irradiating device includes a water-cooling coaxial tube configured with an inter tube and an external pipe enveloping the internal pipe. A light tube is disposed within the internal pipe, and an infrared filter layer is disposed between the internal and external pipes; and an ultra violet filter layer is coated over an external surface of the external pipe so as to filter out an ultra violet light beam having wavelength lower than 320 nm. The breakage of the unit filters resulted from inter pushing with each other or leakage resulted from overlapping of the unit filters can be readily resolved.