To investigate the physical mechanism which causes asymmetric damages in front and rear surfaces, time-resolved dynamics of the fused silica’s surface damage induced by the nanosecond ultraviolet laser is studied using shadow graphic technique. The results show that the damage mechanisms in front and rear surfaces under nanosecond laser interaction with fused silica are absolutely different. Although the plasma and the shock waves in air are relatively high, damage is reduced on the front surface because the plasma shielding limits energy deposition of the remnant pulse .While on the rear surface, two factors aggravate the damage, one comes from the impulsion of the plasma after its absorption of the laser energy, the other comes from the interference between the remnant pulse and the reflected pulse from the plasma. It is instructive to understand the damage mechanism.