The invention relates to the technical field of hydrodynamic cavitation, in particular to a cavitation reactor. The cavitation reactor comprises a power device, a hydrodynamic cavitator and a water tank, wherein a plurality of Venturi type through holes are formed in each cavitation plate on the hydrodynamic cavitator, that is, hole reaming structures are arranged at two ends of each through hole, accordingly, the sectional area of each flow channel becomes small when water flows into the through holes in the cavitation plates from cavitation pipes, the water velocity is increased, and the water flow pressure is reduced; when the water flow pressure is decreased under the saturated vapor pressure of water, cavitation happens, cavitation bubbles are formed, then, the cavitation bubbles flow out of the cavitation plates with the water flow, the water pressure rises, the cavitation bubbles break, shock wave and microjet are formed at the moment the cavitation bubbles break, and high temperature and high pressure are generated. Compared with an existing straight-hole cavitation plate, the cavitation effect is better, and the cavitation efficiency is higher; a plurality of cavitation plates are connected together stage by stage and uniformly distributed in the cavitation pipes, the multi-stage cavitation reactor is formed, the energy utilization rate is greatly increased, and the cavitation effect is improved.