From the perspective of the poverty of stimulus argument and the cognitive development theory, this paper attempts to reveal the mechanism of language acquisition. The poverty of stimulus argument theoretically proves the hypothesis that based on their finite linguistic experience, children can construct their complete language knowledge system. Language acquisition is the process of acquisition of core grammar and of mental representation of vocabulary. Parameter values must be set in the process of children＇s core grammar acquisition. Vocabulary of natural language is stored and processed in the form of conceptual representation, which is associated with syntactic structures by projection rules and bounded by various well-formed rules. The cognitive development theory claims that language acquisition is a development process of universal grammar into particular grammar, which is redescribed at a higher level.
