In A Lesson Before Dying, Gaines makes every character speak for themselves through dialogues, which enables blacks in the marginality to realize and establish their subjectivity. It is also a key concept of Bakhtin's polyphonic consciousness. This paper, therefore, would use polyphonic theory to analyze large-scale dialogue and micro dialogue in A Lesson Before Dying, discussing the marginality behind the dialogues and disclosing author's positive attitudes to the marginality.