Abstract: After Marx's passing, Engels shouldered the theoretical banner of Marxism alone. He combined new developments in capitalism and new practices in the proletarian movement, facing various erroneous trends opposing Marxism, and wrote over 30 prefaces, postscripts, and introductions for re-editions and new editions of their previous works. Engels continued Marx's theoretical tradition of combating various erroneous trends, provided a comprehensive analysis of Marxist theory, staunchly defended and developed Marxism, and developed a unique style of interpreting Marx's teachings in his later years. Engels not only enriched the theoretical achievements of the Marxist ideological treasury and contributed to the construction of Marxist views but also provided a "key" for future generations to understand and grasp his late-life thoughts and theoretical contributions.