Abstract: Based on the KZK equation in frequency domain,the finite difference method is used to analyze the phenomenon of diffraction and calculate the fundamental and higher harmonic sound field of a plane piston source.The research finds that the sound fields of high-order harmonic and fundamental have similar characteristics,and higher order harmonic has less radial diffusion and better sound field.The ultrasonic field characteristic in the air is studied.The di-rectivities and propagation distances of the fundamental and harmonics are analyzed.MATLAB and other mathematical tools have been used for the analysis of the numerical results and simulation.The results provide a reference for the study of the characteristics of ultrasonic sound field.