The event quantifier phrase can occur either before objects or after objects in Mandarin Chinese, which can be expressed as “V+Q+N” and “V+N+Q”. Using the theory of “the mental representations of discourse referents” proposed by Lambrecht (1994), this paper discusses the mechanism of this kind of word order variation. The category of mental representations of discourse referents involves two important concepts――identifiability and activation states. The continuum from unidentifiable referents to identifiable referents reflects the way mankind perceive the world. I exhaustively extracted all instances containing two word orders of “V+N+Xci” and “V+Xci+N” from the closed corpus. After analyzing the cognitive state of all N in each instance and making statistics, I draw the following conclusions: “V+N+Xci” requires N to be highly identifiable or highly activated, while “V+Xci+N” requires that N is relatively unidentifiable or has a low activation degree. This shows that the mental representation state of N is a key factor affecting word order choice. We believe that human cognition is not discrete, but presents a continuum. Therefore the conclusion of this paper expresses not an absolute standard but only a tendency relating to the choice of word order.