Abstract: Indefinite sentence of "ba" is a kind of sentence with "one + classifer + NP" structure as objective. Definite objective of "ba" sentence is fundamentally caused by information structure of "old + new". The nature of definite objective is putting old information with low communicative value forward to spotlight predicate components with high communicative value. As a result of various pragmatic factors, "one + classifer + NP" structure may have the feature of old information, they need to be put forward to spotlight predicate components with higher communicative value.