Half-wave hole has seriously influenced the application of harmonic beam splitters in high-power laser systems. Half-wave holes of the quarter wavelength symmetrical coatings usually caused by overall thickness mismatch and materials dispersion mismatch of the high and low refractive index films are simulated in order to conduct further research on their formation mechanism. Based on the theory of equivalent layers, the equivalent refractive index of a symmetrical design is calculated on Matlab platform. Consequently, the corresponding reflectance envelope is depicted. Through the study on the relation among the equivalent refractive index, reflectance spectrum and reflectance envelope of different designs, the characteristics of half-wave holes such as location, size and change tendency are analyzed in principle. Computational results indicate that thickness mismatch gives rise to equivalent refractive stop-band in the half wave of spectrum. The more the number of layers is, the more serious the thickness mismatch and dispersion mismatch are, the deeper the half-wave hole will be for a harmonic beam splitter.