Abstract: Common in-situ resource utilization (ISRU) technologies of lunar soil are usually conducted at high temperature. Considering ultrahigh vacuum on Moon, unpredictable decomposition and volatilization of lunar soil may be induced, but often is neglected. It will seriously affect the reliability of ISRU technologies. Here, spontaneous phase transformation and volatilization behaviors of typical Highland soil-J (HLS-J) lunar soil simulant under ultrahigh vacuum and high temperature are clarified through thermodynamic calculations and in-situ phase analysis. HLS-J is predominantly composed of feldspars (77.2wt%) and the mineralogical structures are similar to the Apollo 16 highland soil. With the decrease of environmental pressure, complex feldspars phases, i.e. albite, anorthite, and potassium feldspar, become spontaneously decomposable and gaseous simple oxides (such as SiO2, Na2O, Al2O3, et al) are easily produced at a low temperature. Meanwhile, all mineral phases are volatile, particularly under a high temperature and low environmental pressure. The critical volatilization temperatures (CVTs) of various mineral phases to achieve environmental pressure are calculated. It is found that CVTs decrease obviously with the decrease of environmental pressure. The CVT of chromium iron vanadate is the lowest and only about 200°C under lunar pressure (10-12 Pa). The vapor pressure and CVT of feldspars as main component are affected by other mineral phases. For HLS-J lunar soil simulant, CVTs is only about 500 ℃ under 10-12 Pa. This work highlights the neglected decomposition and volatilization behaviors of lunar soil under simulated lunar environmental pressure, which is crucial to develop reliable ISRU technologies.