The Zhaxikang Pb-Zn-Sb polymetallic deposit is one of the most important deposits in the newly recognized southern Tibet antimony-gold metallogenic belt. Compared to the porphyry deposits in the Gangdese belt, much less researches have addressed these deposits, and the genesis of the Zhaxikang deposit is still controversial. Based on field investigation, petrographic, microthermometric, Laser Raman Microprobe (LRM) and SEM/EDS analyses of fluid, melt-fluid, melt and solid inclusions in quartz and beryl from pegmatite, this paper documents the characteristics and the evolution of primary magmatic fluid which was genetically related to greisenization, pegmatitization, and silification in the area. The results show that the primary magmatic fluids were derived from unmixing between melt and fluid and underwent a phase separation process soon after the exsolution. The primary magmatic fluids are of low salinity, high temperature, and can be approximated by the H2O-NaCl-CO2 system. The presence of Mn-Fe carbonate in melt-fluid inclusions and a Zn-bearing mineral (gahnite) trapped in beryl and in inclusions from pegmatite indicates high Mn, Fe, and Zn concentrations in the parent magma and magmatic fluids, and implies a genetic link between pegmatite and Pb-Zn-Sb mineralization. High B and F concentrations in the parent magma largely lower the solidus of the magma and lead to late fluid exsolution, thus the primary magmatic fluids related to pegmatite have much lower temperature than those in most porphyry systems. Boiling of the primary magmatic fluids leads to high-salinity and high-temperature fluids which have high capacity to transport Pb, Zn and Sb. The decrease in temperature and mixing with fluids from other sources may have caused the precipitation of Pb-Zn-Sn (Au) minerals in the distal fault systems surrounding the causative intrusion.