It is investigated that the characteristics of the liquid surface wave amplitude change with the exited depth. Diffraction patterns from the liquid surface acoustic waves(SAW)，which is produced by the exiting needle, were observed. Diffraction patterns from SAW corresponding different exiting depths were found. The relation between the diffraction pattern distribution and the surface acoustic wave amplitude was achieved and this relation was programmed. Then, the dependence of the amplitude on the exited depth was obtained further. It was found that the surface acoustic waves amplitude decreases exponentially as the exited depth increase based on the least square method.