The relationship between the thickness vibration resonant frequency and the size of 1-3-2 piezoelectric composites was studied in this paper. Using the theories of piezoelectricity and elastodynamics, the electromechanical equivalent diagram was established, the thickness vibration modes of 1-3-2 piezoelectric composites were analyzed by electromechanical theory, and the resonant frequency was calculated. The finite element simulation software was used to analyze the relationship among the resonant frequency, anti-resonant frequency and the dimensional variation of 1-3-2 piezoelectric composites in the thickness vibration mode. The results show that the frequency curve decreases when the longitudinal dimension ratio increases, the frequency curve increases when the transverse dimension ratio increases, the frequency curve decreases slowly when the ratio of transverse dimension to longitudinal dimension increases. The frequency curve decreases first and then increases when the thickness of the base increases. The longitudinal dimension ratio has the most significant influence on the overall resonant and anti-resonant frequencies in the four cases.