The invention discloses a device for reducing Nd-Fe-B magnet grain size. The device comprises a clamping device used for fixing samples, an electron beam device used for obtaining pulsed electron beams and a closing device, wherein the clamping device comprises a sliding rail, clamp forceps and a supporting table. The invention further provides a processing method for reducing the Nd-Fe-B magnet grain size. The method comprises steps as follows: adjusting space between the clamp forceps; fixing the samples between the clamp forceps; and the like. According to the device and the processing method, the samples of different sizes can be processed, and the size of sintered Nd-Fe-B magnet grains can be greatly reduced under the processing of the pulsed electron beams; the method has higher efficiency than a traditional method; the method can be highly automatic in a production process; the processing process with high-energy pulsed electron beams is performed in a vacuum environment, so that sample pollution is less, and the processing surface is not prone to oxidizing; processing parameters of the high-energy pulsed electron beams are controllable, and therefore, the microstructure ofNd-Fe-B is easier to regulate and control.