The desensitized explosive PETN film was prepared by physical vapour deposition (PVD) technology,and SEM was used to analyze the microstructure and grain size of PETN film. The relationship between grain size of film and the critical dimension of detonation wave propagation was discussed. The results show that the grain size affects the properties of explosive film, and the decrease of grain size in explosive film helps to decrease critical dimension of detonation wave propagation and microminiaturize explosion element.