This article attempts to analyze the changes and prospects of the Northeast Asian security environment in the era of New-Cold War & multipolarity that has been spreading since the outbreak of the US-China strategic competition and the war in Ukraine. Since the war in Ukraine, changes in the new strategic triangle between the United States, China, and Russia have begun to have an unprecedented impact on the Northeast Asian security environment. This article analyzes how the two factors, the U.S.-China strategic competition and the war in Ukraine, have led to changes in the Northeast Asian security environment. More specifically, the recent changes and prospects of the China-Russia-North Korea triangle and the three bilateral relationships, China-Russia, China-North Korea, and Russia-North Korea. To analyze this research topic, the research design is based on three variables: independent, mediating, and dependent variables. The independent variables are changes in U.S.-China relations and changes in U.S.-Russia relations. The mediating variables are the US-China strategic competition and the war in Ukraine. On the one hand, the confrontation between the U.S.-China relationship led to a strategic competition, and on the other hand, the conflict between the U.S.-Russia relationship led to the war in Ukraine. In addition, the two independent variables can only be influenced by the two mediating variables. In particular, we assume that the domestic political factors of the three major powers, the United States, China, and Russia, played a significant role in causing the two mediating variables. The independent variables and mediating variables work together to promote the Northeast Asian security crisis. As a result, threats to the Northeast Asian security environment have emerged as the dependent variable. The dependent variables are the North Korea-China-Russia triangle, changes in the three bilateral relationships within the triangle, and the confrontation of Northern Triangle versus Southern Triangle. The first mediating variable, strategic competition, has led to several changes in the Northeast Asian security environment: the quasi-alliance of Sino-Russian relations, the restoration and strengthening of North Korea-China relations, the strengthening of trilateral security cooperation between the United States, Japan and South Korea, and the increasing necessity of Sino-Russian-North Korean trilateral cooperation. The second mediating variable, the war in Ukraine, has led to the strengthening of Sino-Russian relations, re-alignment of North Korea-Russia relations, the promotion of the US-Japan-ROK triangular alliance, and the emergence of the China-Russia-North Korea triangular cooperation.