Abstract: Vacuum freezing is an advanced technique for quick freezing and has attracted increased attention worldwide. Compared with traditional freezing, vacuum freezing has advantages of fast cooling rate, short freezing period, and uniform cooling effect. Existing studies on vacuum freezing are limited to liquids, drugs and solid foods. Pleurotus eryngii is rich in protein, polysaccharides and essential amino acids, and a quick freezing can keep it fresh and maintain the nutrients. One concern over the vacuum freezing is its mass loss, but it could be ameliorated by pre-treatments such as adding exogenous water prior to freezing. This paper investigate the efficacy of several pre-treatments, including slicing thickness, blanching and adding exogenous water, on the quality of vacuum -frozen Pleurotus eryngii. The relationship between slicing thickness and water loss ratio after vacuum-freezing was studied, and the impact of adding exogenous water and blanching on vacuum-freezing results was experimentally examined. The results showed that increasing slicing thickness reduced the water-loss ratio, and that the amount of exogenous water that was required to reach a zero-water loss after thawing was negatively correlated to the slicing thicknesses at significant level (P<0.05), with a determination coefficient of 0.98. Adding exogenous water could partially or completely compensate the evaporation loss. When the adding exogenous water was less than 20%, water loss from the Pleurotus eryngii slices without blanching was significantly higher than that with blanching, while the opposite was true when the adding exogenous water exceeded 25% (P<0.05). The loss ratio of the vacuum-frozen Pleurotus eryngii slices after thawing without blanching was significantly higher than that with blanching (P<0.01), indicating that blanching might have significantly reduced the moisture in the vacuum-frozen samples as they were positively correlated to the loss ratio after thawing (P<0.01). The loss ratio after thawing without blanching increased with the added exogenous water, yet no significant difference was found between the loss ratio with blanching when the added exogenous water was more than 10% (P>0.01). The total loss ratio and the relative electrical conductivity of the Pleurotus eryngii slices with blanching were significantly higher than those without blanching, and they all decreased with the increase in the added exogenous water. Microstructure analysis revealed that blanching damaged the cell network in tissues, and adding exogenous water prior to vacuum-freezing could reduce water evaporation from the tissues, thereby ameliorating cellular structure damage. In summary, our results suggested that the optimal pre-treatment for vacuum-freezing the Pleurotus eryngii slices were: slicing at 3 mm of thickness, blanching with 90 °C hot water for 1 minute, and adding 30% of exogenous water.