Chomsky put forth his "A-over-A" theory in his book "Language and Mind", which means if a sentence contains (S… (A...) A…) S (A-over-A) structure, then this sentence can only be transformed on the basis of the larger phrase. In this paper the author puts forth the "Psychological Momentum" theory to analyze the sentences which are ambiguous in meaning according to Chomsky's "A-over-A" theory. In the author's opinion, explaining sentences from the viewpoint of pure grammatical stratum will lead the researchers into a blind alley. As compared with Chomsky's "A-over-A" theory, the "Psychological Momentum" theory is very useful to language researchers, so long as a sentence enters into communication, no matter how ambiguous its meaning may be, it will soon become clear without any ambiguity.