Abstract: In the nonlinearity parameter B/A tomography using the second harmonic wave, it is very important to analyze the ultrasonic field of the transducer, especially the generation of second harmonic wave in the nearfield. In this paper, the theoretical study and experimelltal measurements of the second harmonic pressure field from a circular piston source are performed. And the effect on the nonlinearity parameter tomograaphy is discussed. The results will be used to decrease the error of reconstruction in nonlinearity parameter tomography and bring the ultrasonic diagnosis a step forward.