Considering the characteristics of directeffect and statetransfer among vehicles,the dynamicclassified method is first put forward to discriminate anisotropic and diversity of traffic flows.Based on vehicle travelling characteristics and intervehicles force,matrixes of relatedstate and traffic risk are introduced to analyze dangerous condition.Realtime risk index is then set up after constituting traffic flow riskgrades.Experiments on some freeway sections of highfrequency accident validate the relation between traffic flow risk index and historical accidents.