Abstract: The Sajigoumen porphyry molybdenum deposit is located in the eastern section of the northern margin of the North China Plate. The orebodies occur in the Indosinian monzogranite, dominated by veinlet, stockwork and disseminated molybdenite types of mineralization. Petrography study suggests that the fluid inclusions are well formed in the barren quartz veins before the mineralization stage and the Mo-bearing quartz veins during the ore-forming period, mainly in gas-liquid phase, with minor single-phase and three-phase. The results of microscopic temperature measurement for fluid inclusions show that the homogeneous temperatures in the pre-mineralization stage vary from 196.2 to 390.0℃, with salinity from 5.70% to 17.52% NaCl eq; while those in the mineralization period are from 161.5 to 340.3℃, with salinity of 2.06% - 13.29% NaCl eq. The results of laser Raman spectroscopy show that the composition of fluid inclusions is mainly H₂O in the early stage of mineralization, with minor CO₂ and CO₃^2-. The fluid inclusions in the ore-forming period are two-phase of H₂O and CO₂, three-phase plus CO₂, with SO₂ and CH₄ gas. The change of fluid characteristics from early to later ore-forming stages indicates a continuous decreasing of temperature and pressure, with transformation from oxidation to reduction environment. The ore-forming fluid has experienced boiling and fluid immiscibility. Accompanied by atmospheric precipitation, the magma-controlled hydrothermal porphyry molybdenum deposit is formed under a typical continental collision system.