Arid and semi-arid areas are ecologically fragile regions in China. With the intensification of climate change and human activities, vegetation in this region is undergoing significant changes in recent decades. However, there is still a lack of sufficient understanding of the causes of vegetation change. Based on the GLASS(Global Land Surface Satellite) leaf area index(LAI) and CRU(Climate Research Unit) temperature and precipitation data, this paper studies the temporal and spatial variation characteristics of vegetation in arid and semi-arid areas of China from 1982 to 2017 by using correlation coefficient method and residual analysis method, and analyzes the relative contributions of temperature, precipitation and human activities to LAI changes. The results showed that: 1) Vegetation in most arid and semi-arid areas was significantly improved, and the vegetation improvement was more obvious in summer. 2) Vegetation is positively correlated with temperature and precipitation in most areas of the study area. The influence of temperature on vegetation is mainly manifested in the trend change, while the influence of precipitation is mainly manifested in the interannual change. 3) The simultaneous effects od spring and summer temperature and summer precipitation on vegetation growth in arid and semi-arid areas are significant, while in northern Xinjiang and central Inner Mongolia, vegetation has seasonal lag effect responses to climate factors. 4) Vegetation change is affected by human activities and climate change, and the dominant factors of vegetation change in different regions are various. In the vegetation-improved area, the contribution rate of climate factors is about 59%, and human activities is about 51%; and in vegetation-degraded areas, the contribution rate of climate factors is about -51% and human activities is about -49%. In general, the vegetation in arid and semi-arid areas of northern China has undergone significant changes in recent decades, which is the result of combined effects of temperature, precipitation and human activities.