Abstract: In the background of the great transformation of international situation, and with the purpose of jointly safeguarding the ‘liberal international order’, the defense cooperation between Europe and the US is developing rapidly. This includes improving the cooperation in bilateral and global levels. However, as the EU is pursuing strategic autonomy and defense independence, the transatlantic defense cooperation is facing structural challenges brought by gaps of willingness, goals, interests and capabilities between the two sides. Therefore, although the transatlantic defense cooperation will continue to deepen in the future, it will not develop smoothly. The transatlantic defense cooperation will have negative impacts on China, like the de-stabilizing of international and regional situation needed by Chinese economic prosperity. However, due to the differences in defense positions and capabilities between Europe and the US, the negative impacts of transatlantic defense cooperation on China are limited. Keywords: Transatlantic defense cooperation, Great Transformation, NATO, EU, US, China Jian Junbo and Fang Jiongsheng, "Transatlantic Defense Cooperation in the Era of Great Transformation and Its Implications for China", Security Science Journal, Vol.4 No.2, 2023, pp. 85-115. DOI: 10.37458/ssj.4.2.5. ISSN: 2737-9493