As a novel anodic electrode for Li-ion storage, the cubic Bi4Ge3O12 phase can experimentally deliver a remarkably high reversible specific capacity of 586 mA h g-1 at 200 mA h g-1 with a coulombic efficiency of 99.8% after 500 cycles, and has recently attracted attention for its stable electrochemical performance. Here we calculated its lithiation/delithiation reactions by using density functional theory studies, through the structural changes as the conversion and alloying reaction takes place during the Li-ion insertion and extraction process. The obtained theoretical capacity of Li is 48.75 mol (∼1043 mA h g-1) for 1 mol Bi4Ge3O12. The decomposed Bi2O3 (P3[combining macron]m1) and GeO2 (P3121) in the lithiation process of Bi4Ge3O12 are the active materials to react with the Li atoms via a conversion reaction. Besides Li2O with both Fm3[combining macron]m and Pnma phases, the final lithiation products of Bi4Ge3O12 should include Li3Bi (Fm3[combining macron]m) and Li4.25Ge (F4[combining macron]3m), through the alloying reactions of multi-valence elements of Bi and Ge with Li. Bi and Ge metals are also helpful in the decomposition of Li2O into Li during the delithiation process, increasing the reversibility of the conversion reactions. Our research provides theoretical support to understand the working mechanism of Bi4Ge3O12 and related mixed-metal anode materials.