Abstract: In order to investigate the influence of vacuum environment on the performance of the optical coatings, a low-loss highly reflective (HR) coating sample at 1064nm fabricated by ion assisted deposition was stored in a stainless steel vacuum chamber with pressure less than 1×10^-5Pa. And we observed the variation of absorptance and reflectance of the HR sample. The experimental results show that the reflectance of the sample decreased from 99.9823% to 99.9543% and the absorptance increased from 6.8×10^-6 to 59.5×10^-6 after 335h of storage in the vacuum chamber. By wiping the sample with mixed alcohol/ether solution, the optical performance of the sample is totally recovered. Combined with theoretical analysis, we concluded that a thickness increase of an organic contamination layer on the coating surface and human errors during the experimental procedure are major sources for the performance degradation of the HR coating. The results are helpful to improve the stability of high energy/high power laser optics used in vacuum.