Abstract: On the basis of analysis of sound energy distribution in sound field,the concept of sound streamline which has some similarities to the velocity streamline used in fluid dynamics is offered,and the negative sound intensity is explained geometrically by it in this paper.On the rota-ional character of the active sound intensity,the mechanism of causing the negative sound intensity is also discussed.It is found that the origin of the negative sound intensity is the rotational component of the active sound intensity.