Objectives: To learn that the forming thermodynamic conditions of phenocrysts and original magma of Limu granite in Guangxi. Methods: On the basis of petrography study and classification of crystals of the granite, take the phenocrysts, fine grain zoning plagioclase and idiomorphic crystal quartz, which belong to the melt—crystal group, as the special crystal group for exploring the forming thermodynamic conditions of original magma. By means of phase diagram analysis, to get the forming temperature and pressure of the phenocrysts of the rocks from the second stage granite, further more, to elicit the thermodynamic conditions of original magma forming, and the ratio of Ab to An of source rock. Results: ① The pressure of original magma forming was about 0.43 GPa (equivalent to a depth of about 16km of the upper crust), the melting temperature of eutectic point was about 720°, the final melting temperature was a little higher than 800°, and the ratio of Ab to An of the source rock was greater than7.8. ②The formation of precipitated phases of perthite phenocrysts was aroused by the separating of alkali feldspar solid solution when the temperature of the magma got to the eutectic point of 720 °, before this temperature, alkali feldspar were single- phase and equilibrium with liquid phase. ③The selected phenocrysts did not undergo the staged growth on the way of magma rise, can represent the crystallization of initial magma in deep magma chamber. ④For the magma An—Ab—Or—Q systems which have low calcium feldspar content (An＜5%), the compositions (or components) of the perthite phenocrysts can reflect the partial melting degree of original magma. Conclusions: By means of using phase diagram analysis on phenocrysts, could get the forming thermodynamic conditions of phenocrysts and original magma of Limu granite in Guangxi. This study overcomes the disadvantages made with the calculation of mineral chemical composition, can get the temperature and pressure of crystallization of magma in deep magma chamber and melting temperature and partial melting degree, instead of getting the temperature and pressure of crystallization after magmatic emplacement.