Based on spoken Chinese data,this article discusses the core semantic function of the modal particle bei(呗),and tries to analyze the causes of its function.Through observation,we find that the modal word bei has the function of lowering the speaker's attitude,which is embodied in reducing recognition,expectation and evaluation.This then can be summarized as the function of degraded expression.It is used not only at the propositional level,but also at the discourse expression level.The emergence of the degrading function of bei sentences is affected by the context of negative attitudes.The conventionalization of degrading function of bei is in step with the correlative routinization of negative contexts.The diversity of attitude expression in bei sentences is related to the distribution and function of the discourse.