Based on a review of the historical and current studies on modality, this paper aims at interpreting metaphor of modality and its functions in advertising English according to theories of modality system and metaphor of modality in systemic-functional linguistics with a corpus we have collected. It is pointed out that metaphor of modality, a usual method to scheme modality and allocate modal responsibility, plays pragmatic roles of foregrounding subjectivity and objectivity as well as expressing politeness and text cohesion, hence helping advertisers to establish interpersonal relationship with the readers and to achieve their commercial goals by manipulating them.