Abstract: For its advantages in large radiating area and high radiation efficiency, the stepped circular plate radiator has been widely used in the field of high power ultrasound. The frequency equations of the thick circular plate in free, fixed and simply supported boundary conditions are deduced based on Mindlin theory from the viewpoint of acoustic application, and the frequency equation is numerically solved. The results show that the calculated frequency accords basically with the finite element simulation and experimental measurement. This conclusion provides a reference for the design of the bending vibration radiator of thick circular plate and a basis for the frequency adjustment.