Lunar regolith molding technologies are receiving an increasing attention in lunar exploration program. Most studies are carried out in the air on earth, ignored the effects of high vacuum (~ 10^-12 mbar) on the lunar surface. This paper presents the results of a study aimed at assessing the effect of vacuum on the sintering of low Ti basalt type lunar simulant CLRS-1. The results show that porous sample with density of 1.19 g cm^-3 can be obtained by sintering at 1100 °C under vacuum, which has much lower thermal conductivity (0.265 W m^-1 K^-1) than concrete and other lunar resource derived structural materials. It could potentially be applied as the thermal insulation material for the Moon base construction. The pore-forming mechanism in vacuum was investigated. It was found that evaporation of the products of solid dissolved mineral crystals led to a great deal of weight loss and inhibited the densification during sintering.