The preparation of large hexagonal boron nitride (h-BN) nanosheets with high crystallinity has always been a challenging topic. In this paper, a method for preparing high crystallinity h-BN nanoparticles by microwave-assisted hydrothermal is presented. By means of XRD, SEM, TEM, AFM, Raman and other characterization methods, the validity of the method was proved, and the properties of the nanosheets were characterized. Based on the results of characterization analysis, the mechanism of microwave hydrothermal to prepare boron nitride nanosheets was proposed, which laid a foundation for the preparation of other two-dimensional materials.