This study, based on Prototype Theory, the concept of construal and Stage Model Theory, intends to investigate the cognitive basis, mechanism and features of grammatical metaphor. The data for the analysis comes from interpersonal metaphors in German which are further analyzed in terms of mode, mood and tense. Based on Prototype Theory, congruency and metaphor use reflect the relationship between prototype and periphery with the congruent form as a prototype extending toward the non-central region in the form of metaphor which creates a semantic complex and changes grammatical functions. This type of metaphor can be viewed as periphery in a category. In a metaphor, the construal of speakers' utterance tends to be more subjective and his attitude is the prominence of depiction. Therefore, grammatical metaphor is always accompanied with a relatively higher degree of subjectivity.