The effect of alkali metal modification on g-C ₃N ₄ photocatalyst was systematically studied. The density functional theory is used to explain the microscopic mechanism of different alkali metal modification on improving photocatalytic activity from the various angles of geometry and electronic structure. The results show that the binding energies of alkali metals and g-C ₃N ₄ become weaker and weaker with the increase of the number of extra-nuclear electronic layers of Li, Na, K and Rb atoms, and the influence on the structure of g-C ₃N ₄ is weaker and weaker. The electronic structure analysis shows that Li, Na, and K atoms have an activation effect on g-C ₃N ₄, and Rb atoms have a passivation effect on the surface of g-C ₃N ₄. At the same time, the adsorption of N ₂ molecules on M-g-C ₃N ₄ (M=Li, Na, K, Rb) is simulated, and the interaction mechanism between N ₂ molecules and alkali metals is understood. By analyzing the adsorption energy, structural parameters and electronic properties of N ₂ adsorption, it is found that the influence of Li and Na atoms on N ₂ adsorption is greater. The N—N bond of N ₂ is elongated, and the K and Rb atoms have little effect on N ₂. Alkali metal modified g-C ₃N ₄ is more beneficial for the adsorption of N ₂ molecules than pure g-C ₃N ₄, but as the atomic radius of the alkali metal increases, the adsorption capacity becomes weaker and weaker, and the electron transfer becomes less and less. That is to say, the ability of activating N ₂ molecule decreases in the order of Li, Na, K and Rb.