This paper analyzes the debt crisis faced by the euro zone since the “subprime mortgage crisis” in 2008 and the fundamental contradictions behind the euro crisis caused by it. By introducing the birth of the euro, the article shows that the differences in the competitiveness of enterprises in the euro zone under the unified currency reflect the differentiation of the balance of payments due to the persistent differences in industrial technology and productivity, and bring about the continuous deterioration of the financial situation of deficit countries. The analysis based on labor value theory and unequal exchange theory shows that deficit countries can't recover economic growth by exiting the euro zone and depreciating their currencies. What the euro zone needs is systemic reform to improve the technical level and productivity of economically backward countries. However, the differences in interests between countries and the differences in interests and routes among different groups in Germany, which is in the leading position, made the reform of the euro zone difficult until the crisis broke out.