The graphitic carbon nitride (g-C3N4) nanodots (CN-dots) exhibit properties different from those of g-C3N4 crystal.However, the electronic structure of g-C3N4 nanodots, which determines their properties intrinsically, has not been explored comprehensively. Herein, the many-body Green's function theory is used to analyze the electronic and optical properties of CN-dots; and the effects of size, shape, and functional group on properties were systematically investigated. The large size and the nonlinear shape are effective means to decrease electronic band gap. The increase in the functional group -CHO can make the complex composed of 1D g-C3N4 and 2D g-C3N4 change from type I to type II heterojunction. Different functional groups are related to the absorption edge of CN-dots, while have little effect on the electron-hole recombination rate. These results can provide theoretical support for modifying the properties of CN-dots and further designing CN-dots-based functional materials.