The invention relates to the technical field of heat dissipation, in particular to a piston pressure reduction structure for a water-cooling radiator and a water-cooling radiator. The piston pressurereduction structure comprises a shell and a piston structure, and the piston structure is arranged in the shell. A pressure relief hole is formed in the end, close to one end face of the piston structure, of the shell, and an inner cavity of the shell communicates with cooling liquid of the water-cooling radiator through the pressure relief hole. One end, close to the other end face of the pistonstructure, of the shell is provided with a vent hole communicated with the outside atmosphere. And the piston structure is used for isolating the vent hole from the pressure relief hole. According tothe piston pressure reduction structure for the water-cooling radiator and the water-cooling radiator, the problem of liquid leakage caused by overlarge pressure in the water-cooling radiator is solved.