Plant diversity is the foundation of ecological function of greenland system in expressways and the prerequisites of greenland landscape diversity, which also indicates the level of greening engineering. Applying the theory of plant diversity to the landscape projects on the Ninghuai expressway, this paper evaluates the diversity state of the arbor layer and shrub layer with the approach of α-diversity. It is found that the categories of plants are abundant; the arbor layer of the five interflowing areas evaluated is inferior to the shrub layer in plant species and there exists variations in species index; the variation index Simpson and Shannon-wiener in shrub layer is greater than in arbor layer. The diversity index in shrub layer is relatively high, which indicates high diversity and consistency. The plant diversity in Ninghuai expressway is superior to that of natural communities in the same latitude. The afforestation is close to deciduous broadleaved forests in nature with fewer species than in evergreen broadleaved forests.