The experimental and theoretical studies of damage induced by picosecond laser are presented in the paper.Laser pulses are focused into the interface of silicon and fused silica,which can produce micro channel and micro cavities inside the fused silica substrate which placed on the silicon substrate.No thermal cracks are observed around the micro channel and micro cavities.The diameter of cavities is 1.5~5 μm.The formation mechanism of the damage is analyzed,and a model was established to discuss the formation mechanism and size characteristics of the micro cavities.Quartz absorbs laser energy to form plasma,and the formation and expansion of plasma will produce shock wave,which is limited by surrounding medium and induces microexplosion in a very small volume,and the limited microexplosion forms micro cavities in the material.Finally,the distribution characteristics of micro cavities are analyzed.