Abstract: Walter Benjamin’s thoughts are highly fluid and difficult to summarize as a whole. However, his unique epistemology has a clear developing clue. The monad is not only his own way of thinking, but also an epistemological concept that recurs throughout his creative career. The concept of monad itself thus constitutes a monad from which Benjamin’s totality of epistemology can be deciphered. Research on the concept of monad in his works from different periodsmakes it possible to grasp the overall picture of the development of his epistemology. In “Ursprung des deutschen Trauerspiels ”, Benjamin constructed a new epistemology of truth intended to overcome the flaws of Kantian epistemology, and the idea/monad based on Leibniz’s monadology formed the core of his new epistemology of truth. In “Das Passagen-Werk”, Benjamin, who had undergone a Marxist turn, added a new dimension of historical materialism epistemology to the monad concept. Finally, in “Über den Begriff der Geschichte”, the monad became the key medium at the historical materialist’s psychological level to move them from historical awareness to revolutionary practice.