<p>We propose the term “Southern Identity” to encompass the concepts of the Third World, developing countries and the Global South, establishing a logical counterpart to Western Identity. The concepts of the Third World, developing countries and the Global South arose within a brief span of time, yet their usage frequency has varied substantially across different historical periods and international contexts. In contrast to Western Identity, Southern Identity seems to feature a certain degree of conceptual fluidity. To address this issue, we first sort out the debates on the stability and fluidity of identity in existing research, and theoretically explain the causes of the differences in the stability and fluidity of collective identities in the international community. Subsequently, we analyze the three-stage evolution of South-South Cooperation in the situation of post-war history: the Third World constructed its political identity through the Non-Aligned Movement yet witnessed its dissolution after the end of the Cold War; developing countries focused on the pursuit of economic equity, and their identity recognition faces dual challenges: external pressures from the West and internal development disparities; the Global South strengthened its subjectivity and consolidated Southern Identity by virtue of its enhanced economic strength and the practice of diverse roles. We argue that Southern Identity is a unified and continuous whole, whose stability hinges on the joint underpinning of its own values, subjectivity and institutionalization.</p>