The Piaotang tungsten deposit is a large vein type W polymetallic deposit in southern Jiangxi Province, assemblage of W and Sn is one important feature of this deposit. Based on detailed petrographic observations and the means of Fluid Inclusion Assemblage (FIA), we carried out microthermometric and Raman microspectroscopic studies of fluid inclusions in quartz and coexisting cassiterite in quartz veins which tin mineralized significantly. The results show that, the physical and chemical conditions of cassiterite and quartz formation are not consistent, and the fluid evolution process related to the two types of minerals are also significantly different. The primary fluid inclusions in cassiterite reflect the actual temperature and pressure conditions of cassiterite formation. The fluid related to tin mineralization for NaCl H2O system with high temperature and medium to low salinity. The study indicate that, the fluid related to tin mineralization may be mainly from crystallization and differentiation of magma, and the cooling of fluid is the main mechanism of tin precipitation in the fluid.