The invention relates to the field of nuclear auxiliary cooling water of 1000 MW nuclear power plant and provides an overwater washing and blowing device for capacity expansion of a spent fuel pool and an overwater washing and blowing method. The device comprises an annular pipeline, a first pipeline, a second pipeline and a nozzle assembly, wherein the first pipeline and the second pipeline are communicated with the annular pipeline, liquid or gas is conveyed into the annular pipeline from the first pipeline, the liquid or gas enters the second pipeline which vertically extends downwards through the annular pipeline under the action of pressure, and then the liquid or gas is sprayed outwards through the nozzle assembly on the second pipeline so as to achieve the aims of flushing and accumulated water blowing. The device integrates the flushing function and the accumulated water blowing function, and an old framework can be effectively flushed to reduce the radioactivity of the old framework; meanwhile, the washed framework can be quickly aired, and the capacity expansion efficiency of the spent fuel pool is greatly improved; the device is simple in structure, easy to operate and low in manufacturing cost; the operation process of the overwater washing and blowing method is simple and convenient, and the working efficiency is high.