As a major emitter of carbon dioxide (CO₂), China is committed to achieving carbon neutrality (CN). However, unilateral climate policies risk carbon leakage through the relocation of emission-intensive industries, potentially undermining their intended benefits. Herein, we couple an integrated assessment model with a simple climate model to quantify the carbon leakage associated with China’s CN by 2100 and assess its impact on global warming. The results show that China achieving CN will lead to carbon leakage of 6.9% to 14.9%. While the emissions-level changes appear modest, the leakage leads to an increase in CO₂ concentrations, rising from 2.1 ppm in the target year to 7.7 ppm by 2100. The global cooling effect of China’s CN is 0.15-0.19 °C, with leakage weakening this effect by 0.02 °C. Increased carbon sink capacity mitigates 8.9%-11.3% of the warming caused by leakage, and synergistic reductions in methane and nitrous oxide emissions offset over 70.5% of the leakage-induced warming. Therefore, solutions to carbon leakage should go beyond merely preventing increased emissions and focus on offsetting the broader climate impacts caused by leakage. These findings underscore the critical importance of addressing carbon leakage in climate policy design to mitigate the risks of global warming and maximize the benefits of CN efforts.