Abstract：At present, there are diversified forms of security cooperation mechanisms coexisted in Southeast Asia. On the one hand, all kinds of security mechanisms are developing towards a form characterized by diversification, sectorization and specialization. On the other hand, boundaries among security mechanisms become more indistinct and it shows a tendency to overlap and competition, which form the phenomenon of “fragmentation of security cooperation mechanism”. Fragmentation of security cooperation mechanism in Southeast Asia is mainly displayed by the mechanisms diversification, mechanisms complication and mechanisms disordercompetition. Imbalance of power, diversification of interests and reduction of identification are the main reasons for fragmentation of security cooperation mechanisms in Southeast Asia. At the same time, fragmentation diminished the institutionalization level of regional security cooperation, exacerbated the complexity of security cooperation mechanisms and weakened the dominance of ASEAN in regional security governance, which poses a severe challenge to security governance in Southeast Asia. Therefore, China should make efforts to improve the institutionalization level of security governance in Southeast Asia by becoming the “agendasetting nation” and propelling the CICA process to promote the coordination among existing security cooperation mechanisms. Also China is supposed to support the comprehensive security cooperation mechanism, especially join and support the interregionalism with ASEAN as the core and actively practice the Asian security concept featuring common, comprehensive, cooperative, and sustainable security so as to ensure the dominance of ASEAN in Southeast Asia.