Ladder type disc acoustic radiator is widely used in high power ultrasonic field. The frequency equation of stepped thickness disk in free boundary condition are derived based on Mindlin theory from the acoustic point of application and The frequency equation was numerically solved,At the same time,the effects of geometric parameters on the frequency is analyzed. The results show that when the other parameters are constant,the frequency increases with the increase of the disk and the base ring thickness,and decreases with the diameter increases,increases with the increasing diameter. The test results of the calculated frequencies and finite element simulation and experiment is consistent with the basic. Design of bending vibration radiator the conclusion of the thick disc provides the theory reference and the frequency of debugging basis.