Based on detailed petrographical and mineralogical compositional features of biotite from the granodiorite porphyry around the Jinjiwo deposit in the Jiurui ore district, the authors not only established the physicochemical conditions of the mineral formation but also explored its significance for rock and ore formation. Major element composition of the biotite phenocrysts of the samples was measured by the electron probe micro-analyzer (EPMA). The Ti content and the Mg/(Mg+Fe) ratios of the biotite from the Jinjiwo granodiorite porphyry are 0.44~0.47 apfu (atoms per formula unit) and 0.58~0.62, respectively, and the compositions of these biotite are plotted in both the domain of the primary magmatic biotite and that of the Mg-rich biotite. These features suggest that the biotite phenocrysts of the samples are all eastonite, and that these analyzed biotites display the Mg-rich and Fe-poor characteristics. The oxidation coefficient and the MF values of biotites are 0.31~0.67 and 0.38~0.41, respectively. Biotite has 13.27%~14.28% MgO, 3.59%~8.93% K₂O, 0.26%~3.28% Na₂O, 15.74%~17.30% FeO, 3.93%~4.19% TiO₂, and 13.52%~14.19% Al₂O₃, respectively. It contains about 2.00~2.24 apfu of tetrahedral Al^Ⅳ and 0.15~0.41 apfu of octahedral Al^Ⅵ. The TAl content of biotite can be used to figure out the solidification pressure of the granitoid. Considering the composition of biotites together with experimental results of the former research, which falls on or above Fe₂O₃-Fe₃O₄ buffer in the Fe³⁺-Fe²⁺-Mg diagram, the authors consider that the biotites from granodiorite porphyry in the study region were crystallized under the condition of temperatures 734~748℃ and pressures 71~89 MPa, corresponding to solidification depth 2.7~3.3 km. The above characteristics suggest that the Jinjiwo granodiorite porphyry belongs to high potassium Ca-alkaline series, showing the typical syntexis type (or I-type) granitoid derived from a crust-mantle mixing source, and that the pluton was formed in a relatively high oxygen fugacity environment, which was beneficial to copper mineralization.