[Background] The characteristic of the plant community structure and the plant diversity in the course of natural restoration of vegetation is a scientific basis to construct artificial vegetation in semi- arid loess area of northern Shaanxi, which is the typical fragile ecological region. [Methods] The paper explored what was the influence mechanism of the topography on natural vegetation in this region, for Hegou watershed in Wuqi County of Shaanxi Province as an example. Based on the vegetation survey in 50 sampling plots, the community structure and the plant diversity were nalyzed from three aspects of landform , slope and gradient. [Results] The results showed that the plant community was still mainly herbaceous and the zonal vegetation type was Artemisia sacrorum + Stipa bungeana communities in this watershed, which had been fenced for 15 years. Not only were there Populus simonii and Salix matsudana on gully bottom, but there were also three trees of P. simonii, Ulmus pumila and Ameniaca sibirica and six shrubs of Buddleja alternifolia, Prinsepia utilis, Spiraea trilobata, Caragana korshinskii, Caragana microphylla and Periploca sepium on some micro-topographies of slope aspect with favorable water condition, i. e. , gully, collapse and platform. On different landforms, the order of the richness index was valley slope > gully slope > top valley > gully bottom; that of the diversity index was valley slope > top valley > gully slope > gully bottom; and that of the evenness index diminished from top to bottom. On different slopes, both the diversity and evenness indices were in the same order of size, i. e. , shady slope > semi-sunny slope > semi-shady slope > sunny slope, but the richness index was in the order of semi-shady slope > semi-sunny slope > shady slope > sunny slope. Moreover, with the increase of gradient, the richness index decreased gradually, and the evenness and diversity indices were both of “ S冶types, and its maximum value was at 32 of gradient. [ Conclusions] Therefore, in summary, in the process of artificial vegetation of restoration and reconstruction in this region, we should figure reasonable vegetation layout according to the differences of the restoration ability of natural vegetation on different terrains. In order to enrich the structure and optimize the ecological function of the plant community, the appropriate proportion of trees or shrubs could be configured on gully bottom and some micro-topographies of slope aspect, in where water condition meets the need of trees and shrubs. But on sunny slope, especially the steep slope, it should be the first task to protect and restore the herbaceous community. In addition, to speed up the process of vegetation restoration in the loess area of northern Shaanxi, some positive trees or shrubs should be properly distributed based on artificial measures, such as fish scale pit of collecting water or level bench of reducing gradient.