Since the mid-2010s, China has gradually but explicitly altered its position in the process of global climate negotiations, from a vetoing to a leading nation. Experts in the International Relations field have commented that the main reason is China’s aspiration to become a global power, using the “environment” as one of the effective means to consolidate the nation’s soft power and reinforce multilateral diplomacy. However, a political interpretation (seizing the opportunity to secure hegemonic power in the process of regime strengthening) does not adequately explain the complexity behind China’s changing role in this transitory period. In this regard, a question arises: “Why and how has China undergone such an evolution, particularly in the arena of climate regime?” I suggest broadly five factors along with detailed discussions. The main factors include both external (e.g. regime interplay between climate and development aid, Sino-U.S. relations, global green technology market) and internal (China’s ecological modernization). The paper argues that all those factors interacted with one another in a relatively effective way during the current transitory period of China moving toward an environmentally sustainable society. While each of the above-mentioned factors is driven by different and multi-layered forces behind them, the commonly applicable driver cutting across all factors is “the state’s strategic use of transitory duality.” The paper concludes with a brief discussion on the implications of China’s increasing role in global ecological development.