The invention belongs to the field of underwater laser machining, and particularly discloses a laser-air water coaxial jet coupling head and an underwater laser machining device. The coupling head comprises a laser adjusting device, a high-pressure water cavity wall and an auxiliary air cavity wall which are arranged coaxially in sequence. An energy transmission optical fiber splice, a collimatinglens, a focusing lens, a laser window, a water jet nozzle and an air water coaxial jet nozzle are all located on the axis of the coupling head. Laser beams introduced from the energy transmission optical fiber splice are jet out after passing through the collimating lens and the focusing lens in sequence, form light-water coupling jet flow with water jet flow, and then form coaxial jet flow withair jet flow, then the coaxial jet flow is sprayed out, and then a machined component is machined. The invention further discloses an underwater air water coaxial jet flow auxiliary laser machining device. According to the coupling head and the auxiliary laser machining device, requirements for the underwater laser focusing control precision can be lowered, influences of seawater on the laser beams, high-pressure jet water beams and the machining quality can be isolated, and meanwhile, the advantages that the device is simple in structure, low in cost, easy to operate and the like are achieved.