Our density functional theory calculations show that silicon doping in g-CN (SiC3N3) can improve the electrochemical performance of g-CN as an anode of alkali metal-ion batteries and solve the problems of too high adsorption ability and migration energy barrier commonly found in porous carbon nitride. The stability of SiC3N3 was verified by molecular dynamics simulations and phonon spectroscopy. Elastic constant calculations revealed that the Si doping in g-CN can improve its mechanical properties. Specifically, Li/Na/K has a suitable adsorption capability (-0.71/-0.52/-0.98 eV) and a lower migration barrier (0.73/0.43/0.21 eV) on SiC3N3, where the barrier of a single Li-ion is the lowest among the doped porous carbon nitride materials studied so far. Moreover, SiC3N3 exhibits a high theoretical capacity (253/1512/1512 mA h g-1) and a low open-circuit voltage (0.48/0.18/0.31 V) for Li/Na/K ion batteries. Compared with B-doped g-CN previously studied, Si doping can more effectively improve the electronic conductivity of g-CN owing to greater charge transfer between Si and g-CN; the migration energy barrier of alkali metal ions on SiC3N3 is reduced more significantly due to its puckered structure instead of a planar structure; and the capacity of SiC3N3 is nearly doubled for alkali metal ion batteries because it has more feasible adsorption sites for alkali metals. These results suggest that Si-doped g-CN can be a universal anode material for alkali metal ion batteries.