In the Critique of Hegel’s Philosophy of Right, Marx went through an evolutionary process in his thought on the relationship between civil society and political state. The main turning point was in Paragraph 303 on page XXIII. Before this point, Marx did not take civil society and political state as two corresponding and opposite categories, not to mention a very deep research on their relationship. But after this point, Marx began to focus on civil society and political state, and made intensive and detailed explorations on their relationship in a quite wide historical perspective. Thus he made a dramatic leap forward. Through historical investigations, we hold that Kreuznach Notebooks should be regarded as the cause for this leap forward.