Abstract: In most study on coated contrast agent microbubbles, the single-microbubble model is used as the theoretic basis, however the actual measurements are executed under microbubbles group circumstances. In this article, our experiments were strictly controlled in order to acquire the backscatter signals from single microbubble in pulse ultrasonicfield, and then the short time Fourier transform was applied to study the instantaneous nonlinear characteristics, which were not reported in others' former work.