The utility model discloses an underwater illuminating lamp, in particular to an illuminating lamp which can be used underwater. The underwater illuminating lamp comprises a lamp casing which is in a tube shape; the upper end and the lower end of the lamp casing are respectively provided with a transparent cover and a watertight socket connector; the inside of the lamp casing is sequentially provided with a driving circuit board, a cooling plate, a luminous element and an optical collector from bottom to top; wherein, the transparent cover and the watertight socket connector are matched with the lamp casing in a sealing mode; the optical collector is a short conical cylinder, the small end of the optical collector is covered around the luminous element, and the upper end is abutted to the periphery of the transparent cover; and the luminous element is an LED. The underwater illuminating lamp has low power consumption, can improve the energy utilization ratio as well as prolong the voyage and underwater working time of a deep-sea submergence vehicle, and is suitably applied to a deep manned submersible.