Input theory as a theoretical foundation in language teaching plays an important role in SLA. Though a wealth of re?search has been done by linguists to demonstrate the importance of language input in SLA, little has been written about the type and amount of language input for successful SLA, especially its processing model while acquiring a second language. This paper first discusses the Krashen's input hypothesis in language learning, and then an introduction to Chaudron's processing model of in?put is made. In the final part, the author explains the acquisition process based on word acquisition and grammar acquisition and concludes that in the process of acquiring a second language, the language learners reconstruct a new cognitive model by taking in consistent comprehensible language input.