In high vacuum environment of 10-3 Pa, using 355 nm pulsed ultraviolet(UV) laser we repeatedly raster scanning fused silica at energy density of under zero probability damage threshold, with the purpose of studying the change in anti-damage ability of fused silica after receiving different UV laser irradiation dosages in high vacuum. The damage measured after irradiation and increasing of damage pit indicate that the anti-damage ability of fused silica obviously drop, the degree of depression is more closely correlated with the irradiation time, while the influence of irradiation energy density is weak. The measurement of fluorescence excitation and X-ray photoelectron spectros copy proves that the increase of oxygen-deficiency in fused silica is the main cause, and the degree correlates with the received UV laser irradiation dosage.