The present study, by analyzing the interactions of verbs and constructions in Goldberg's construction grammar, is an exploration of the cognitive mechanism of "zhidao...de...bu zhidao...de" within the theory of metaphor postulated by Lakoff. The paper holds that "zhidao...de...bu zhidao de..." is a complex sentence with zhidao de and bu zhidao de as conjunctions, the consturctional meaning of which is "P is like Q", that is a description of attributes of a related person, thing, action or event. it is observed that the cognitive mechanism of "zhidao...de bu zhidao...de" is a metaphor. The operation of the metaphor is to map the atrribute(s) of Q onto P. The trigger of this metaphor is the surprise of the speaker with the attribute(s) of the related person, thing, action or event. The necessary condition of the metaphor is the metaphorical presupposition in the source domain.