Vacuum window (VW) damage is the bottleneck restricting the load capacity of high-power laser device, and the influence of vacuum environment is worthy of attention. Firstly, based on the fluororubber ring seal, the laser-induced damage threshold (LIDT) of fused silica VW irradiated by the 351 nm laser was measured. It is found that the LIDT of the VW decreases by about 50% after the close contact with the fluororubber ring. After repeated extrusion, the influence of the fluororubber ring on the LIDT of the VW is significantly weakened. On this basis, the LIDTs of the VW under atmosphere, 10³ Pa and 10^-2～10^-1 Pa are compared. The results show that the LIDT of vacuum window decreases significantly with the decrease of air pressure, and the LIDT does not increase after the air pressure rises again. In addition, the LIDT of the VW under different air pressure is tested based on indium ring sealing, while the effect of air pressure difference on the LIDT is not found. Comparing the results of the two sealing materials, it is considered that the LIDT decreasing of the VW is mainly caused by the release of organic contamination from fluororubber ring and the release of organic matter will be intensified under low pressure.