The phenomena of acoustic emission in LN:Fe crystals nave been observed during the process of light induced quasi-breakdown. It is found that the super high frequency acoustic signal has been modulated by the low frequency acoustic wave which was introduced into the crystals, the frequency of which increases with the increasing of intensity of incident light and the period of the breakdown is the same as the jump one of the photovoltaic current and the light induced change of refractive index. It is caused by the strain effect of crystal induced from jumped space change field.