The formation conditions of topaz seem to be the key for the understan- ding of the topaz-bearing rock genesis.In order to quantitatively explain. the petrologic role of topaz, the authors used the thermodynamic properties of topaz and coexisting minerals (andalusite, muscovite,K一feldspars, albite, fluorite and quartz) as well as fluid components(H2O, HF, KF and NaF)to calculate the mineral equilibria of related reactions in the granite-H2O-HF system, which are expressed in logaHF-logaKF，logaHF-logaAn, logaHF-T and T-P diagrams. The major results are: 1 .An increase in HF activity of the granite-H2O-HF system is favorable to the formation of topaz. Within a specific range of HF activity, topaz may coexist with alkali-feldspars and muscovite. However, the condition of higher HF activitZr can only stablize the topaz+quartz assemblage. 2 .VVith the falling of temperature, the stability field of topaz is expa- nded relative to the stability fields of andalusite, muscovite, K-feldspar and albite, and the stability field of muscovite is expanded relative to the fields of andalusite and K-feldspar. 3.Fluorination reactions between plagioclase and HF produce the fluo- rite士topaz+quartz assemblage. They buffer the anorthite component activity it plagioclase. The system of higher af or aHF can only stablize albite along with topaz. 4 .Relative to the F-free granite system, the pressure lower limit: of muscovite stability descends and its temperature upper limit ascends in the F-bearing granite system. In the presence of topaz, muscovite as a magmatic mineral can be stable above the solidus of F-bearing granite under the pressures as low as(1-2)×108Pa. The above calculated results can be used to explain the formation condi- tions of topaz-bearing granitic rocks and their extreme differentiates a.nd the formation mechanism of alteration, zones in porphyry Mo, Sn deposits.