Abstract: The shock formation in a nonlinear ultrasound beam by solving KZK equation using frequency-domain numerical method is investigated. The results reveal that besides the nonlinearity and absorption, the diffraction is another important factor that affects the shock formation of an ultrasound beam. It also reveals that the shock formation distances of piston field are much different under different diffraction conditions. At last, the waveform of sound pressure at the formation of shock and the spatial distribution of shock formation are also presented and discussed.