Abstract: Abstract: Ultrasonic atomization had been paid so much attention as a new spraying technique recently. With such advantages as small droplets, uniform size distribution, high roundness, large atomization quantity and low liquid delivery pressure, ultrasonic atomization is widely used in aeroponic, agricultural humidifying fields, reagents atomization treatment, semiconductor etching and so on. According to different the working frequency, the current ultrasonic atomizer mainly includes high-frequency (working frequency is greater than 1 MHz) and low-frequency (working frequency is in the range between 20 kHz and 100 kHz). Application of high-frequency ultrasonic atomizer was highly limited for such disadvantages as low reliability, high driving voltage, short continuous working time, high energy consuming, failing in atomizing high viscosity liquids, small and unstable atomization amount. Above all, chemical structures of liquids atomized by high frequency ultrasonic atomizer would be changed, so this kind of atomizer was just suitable for atomizing water. Current actuators of low-frequency ultrasonic nozzles were mainly made of Langevin piezoelectric actuators, which included piezoelectric discs, front covers and rear covers. Such disadvantages as high driving working voltage, low efficiency, severe heat, large volume and uneven distribution of droplets limited this kind of low-frequency ultrasonic nozzles to be spread. In order to develop a low frequency ultrasonic nozzle with such advantages as low driving working voltage, high working efficiency, small feat, fine and uneven distribution of droplets, a novel low frequency ultrasonic nozzle, whose actuator was an axial symmetry bending composite piezoelectric actuator, was proposed and designed in this paper. While the core component of this ultrasonic nozzle, namely, an axial symmetry bending composite piezoelectric actuator was composed of a piezoelectric ceramic ring and a metal disc. At the present, the fundamental vibration frequency of this kind of actuator was mainly applying Rayleigh-Ritz theory based on the minimal energy principle to search an approximate value without considering electronic-mechanical coupling. Because Rayleigh-Ritz theory ignored the effect of t electronic-mechanical, the calculation result error was too bigger. At the same time, this method was so complicated that it's not suitable for engineering calculation. So it's necessary to find a simple calculation formulation for engineering application. In order to find out how key structural parameters of axial symmetry bending composite piezoelectric actuators influence on their fundamental frequencies, a virtual testing system based on finite element method was established. In this virtual testing system, element Solid 98, an tetrahedral coupled-field solid element in ANSYS software was used to mesh the prototype of axial symmetry bending composite piezoelectric actuator. Combined with an orthogonal experimental design method, an orthogonal protocol of four factors and five levels was proposed. Based on virtual test data, a regression model between fundamental frequency and key structural parameters of piezoelectric vibrators (significance level α=0.05) was established. The regression model indicated that: i) P-values of outer diameters, inner diameters of nozzle's piezoelectric discs and diameters of metal discs were 1.2×10-8、9.97×10-6、2.093×10-3 respectively. It indicated that fundamental frequencies of axial symmetry bending composite piezoelectric actuator were affected by outer diameters, inner diameters of nozzle's piezoelectric discs and diameters of metal discs highly significantly; ii) P-value of piezoelectric disc thickness was 0.012813. It indicated that fundamental frequencies were influenced by piezoelectric disc thickness significantly; iii) outer diameter, inner diameter, thickness of nozzle's piezoelectric ceramic and diameter of metal disc influence frequencies of those actuators in turn. Tests of the nozzle's acoustic impedance were conducted too by PV 70A, an instrument used for measuring piezoelectric part's acoustic impedance parameters. Compared fitting results with test results, fitting errors are nearly all less than 5%, so the regression model was verified. This regression model made it easier to optimize axial symmetry bending composite piezoelectric actuator design.