This paper took a typical plateau river valley city - Xining City as an example and quantified the spatiotemporal patterns of urban expansion and vegetation cover from 2000 to 2015. We then analyzed the impact of urban construction on vegetation coverage and the quality of the ecological environment. The results showed that: (1) From 2000 to 2015, the area of build-up lands increased by 25.99%, whereas the urbanization rate was 4.76%. Meanwhile, the urbanization levels of different districts and counties varied widely; (2) There was a slight but insignificant increasing trend in vegetation coverage (P < 0.57). The city was dominated by Medium-High and High vegetation coverage, in which spatially distributed characteristics are "east-west low and north-south high". The vegetation coverage in the area of returning farmland (grazing) to forest (grass) has increased significantly (P < 0.01), suggesting the restoration project achieved significant effects; (3) On the district or county scale, the urbanization change rate and the medium-high coverage vegetation change trend showed a significant negative correlation (P < 0.01). Different degree of urban development and location conditions are the main factors affected the medium-high coverage vegetation; (4) On the grid-scale, the area of low-cover vegetation in the built-up area and its surrounding areas showed a significant increase (P < 0.01), indicating that the quality of the ecological environment in urban areas has improved; The research results are of great significance for protecting and improving the quality of the ecological environment of Xining City and for rationally advancing urban construction.