Abstract: The South China Sea has become a focal point for major-power competition and regional security cooperation. In recent years, intelligence collaboration among the United States, Japan,and the Philippines in this region has accelerated and exhibited a trend of institutionalization. Based on the “asymmetric alliance theory”, this paper explores the threat narrative, major measures, potential impacts and corres ponding responses of thd trilateral intelligence cooperation among the three countries. Research reveals that the intelligence interaction among the U.S., Japan, and the Philippines exhibits a “dual-layer asymmetry”, characterized by an alliance structure where the U.S. leads, Japan assists, and the Philippines cooperates. Under this “asymmetric alliance” frmework, the three parties employ a meticulously crafted narrative of threats to frame the South China Sea issue as a “security challenge” requiring collective action, thereby providing a so-called legitimizing basis for their intelligence collaboration. The main measures of the trilateral intelligence cooperation in the South China Sea are reflected in intelligence working mechanisms and coordination, military and non-military integrated intelligence interactions, the deployment of intelligence technologies, as well as maritime situational awareness and joint military operations. The intensified intelligence cooperation among the U.S., Japan and the Philippines in the region has undoubtedly strengthened the deterrent effect of the U.S. alliance system and heightened tensions in the area, yet their strategic divergences and domestic political constraints have simultaneously restricted the stability of such cooperation.

Key words: U.S.-Japan-Philippines trilateral cooperation, “asymmetric alliance”, intelligence cooperation, “threat narrative”, South China Sea issue