Two-dimensional single-layer C3N has attracted extensive attentions currently owing to its excellent mechanical and electronic properties. In this work, as a potential anode material in lithium-ion battery, the pristine and defect-containing C3N are systematically studied base on first-principles calculations. The results show pristine C3N might has great potential in the application of lithium-ion batteries due to its excellent stiffness (Young's modulus is 364.33 N m(-1)), high storage capacity (1071.56 mA h g(-1)), good electronic conductivity (bandgap is 0.39 eV), and lithium migration capability (energy barrier is 0.27 eV). We further investigate the adsorption and diffusion mechanisms of lithium ion in C3N with and without vacancy defect and find out as an anode material, it is vital to control defects of C3N to improve the performance of lithium ion batteries. These studies could deepen the understanding of perfect and defect-containing two-dimensional materials and provide a guidance for the application of CN-based material in lithium-ion batteries.