Fabricating heterojunction photocatalysts is an important strategy for speeding up the separation rate of photogenerated charge carriers, which is attracting greater interest. However, the choice of three factors, individual materials, band offsets, and effective interfaces, is still important for fabricating efficient heterojunction photocatalysts. Herein, efficient g-C₃N₄/Zn₂GeO₄ photocatalysts with effective interfaces were designed by controlling the surface charges of the two individual materials inside the same aqueous dispersion medium, making use of the electrostatic attraction between oppositely charged particles. The g-C₃N₄/Zn₂GeO₄ heterojunction with opposite surface charge (OSC) showed higher visible-light photocatalytic activity for degradation of methylene blue than those of pure g-C₃N₄, pure Zn₂GeO₄, and the g-C₃N₄/Zn₂GeO₄ with identical surface charge (ISC). The investigation of the light absorption spectrum, adsorption ability, and photocurrent responses revealed that the improved separation of photogenerated carriers was the main reason for the enhancement of the OSC g-C₃N₄/Zn₂GeO₄ sample's photocatalytic activity. By combining with theoretical calculations, we investigated the microscopic mechanisms of interface interaction and charge transfer between g-C₃N₄ and Zn₂GeO₄. The photogenerated electrons in the g-C₃N₄ N 2p states directly excited into the Zn 4s and Ge 4s hybrid states of Zn₂GeO₄. The strategy of designing and preparing a g-C₃N₄/Zn₂GeO₄ composite catalyst in this work is very useful for fabricating other efficient heterojunction photocatalysts.