The Fucheng composite pluton is exposed in an area of about 850 km2 and intruded into Sinian Cambrian strata with a distinctive thermometamorphic aureole of hornstone. The first stage major phase granite is coarse-medium-grained porphyritic biotite granite, whose major minerals are quartz, K-feldspars, plagioclase and lesser biotite and muscovite, and whose accessory minerals are zircon, apatite, topaz, tourmaline etc. The second stage additional phase granite is medium-fine-grained two-mica granite in the form of a small stock, whose major rock-forming minerals are quartz, K-feldspars, plagioclase, muscovite and biotite. As for petrochemistry, the indices of alumina saturation ［ACNK=Al2O3/(CaO+Na2O+K2O)］ of Fucheng pluton are higher than 1.0 (ACNK=1.22 on average), indicating that the Fucheng pluton belongs to strongly peraluminous granite. The whole rock Rb-Sr isochron ages of the first main-stage granite are 226～203 Ma, whereas the zircon U-Pb ages are 231～239 Ma, implying that the granite is an intrusive product of late Indosinian magmatic cycle. Based on microscopic observation and electron microprobe analyses, the authors recognized three kinds of muscovites, namely primary, metasomatic and secondary muscovites, in the Fucheng strongly peraluminous granite pluton, and their average crystallochemical formulas are respectively as follows: K1.62Na0.06Fe0.32Mg0.39Ti0.02Al4.89Si6.54O10(OH)4 (primary muscovite), K1.55Na0.07Fe0.43Mg0.24Ti0.03Al4.96Si6.50O10(OH)4 (metasomatic muscovite), K1.51Na0.07Fe0.27Mg0.21Ti0.00Al4.98Si6.65O10(OH)4 (secondary muscovite). According to the crystallizing sequence of main rock-forming minerals (plagioclase→K-feldspar→biotite→muscovite→quartz) in combination with an analysis of muscovite and biotite stability curves and syntectic granite solidus, it is considered that the metasomatic muscovite was formed by the replacement of early crystallized biotite in the crystallization process of the granite melt. Moreover, as uhe crystallization temperature of the muscovite was higher than the sollidus temperature of the granite melt (about 650℃), it is concluded that the metasomatic muscovite belongs to primary muscovite. A synthetic method for the discrimination between primary and secondary muscovites based on macroscopic petrological and petrochemical characteristics as well as micro-petrographic features is suggested in this paper.