Development of lithium-ion batteries (LIBs) using transition metal oxides (TMOs) becomes more attractive these days, due to their higher specific capacities, better rate capability and high energy densities. Herein, the in-situ growth of an advanced mesoporous CuO/O-doped g-C3N4 nanospheres is carried out in two steps hydrothermal at 180 C and annealing in air at 300 C. When used as anode material, the CuO/O-doped g-C3N4 nanospheres achieve a high reversible discharge specific capacity of 738 mAhg-1 and a capacity retention ̴ 75.3% after 100 cycles at current density 100 mAg-1 compared with the pure CuO (412 mAhg-1, 47%) and O-doped g-C3N4 (66 mAhg-1, 53%). Even at high current density 1 Ag-1, they exhibit a reversible discharge specific capacity of 503 mAhg-1 and capacity retention ̴ 80% over 500 cycles. The excellent electrochemical performance of the CuO/O-doped g-C3N4 nanocomposite is attributed to the following factors: (I) the in-situ growing CuO/O-doped g-C3N4 avoids CuO nanoparticles aggregation, leading to the improved lithium ions transfer and electrolyte penetration inside the CuO/ O-doped g-C3N4 anode, thus promoting the utilization of CuO; (II) the porous structure provides efficient space for Li+ transfer during insertion/extraction process to avoid large volume change; (III) the O-doping g-C3N4 decreases its band gap, ensuring the increased electrical conductivity of CuO/O-doped g-C3N4; (IV) the strong interaction between CuO and O-doped g-C3N4 ensures the stable structure during cycling.