Abstract: Arid and semiarid regions are ecologically fragile regions in China. Over the last decades, the vegetation in these regions has changed substantially due to the intensifying climate change and human activity. However, the causes of the variations in vegetation are unclear. Based on the Global Land Surface Satellite (GLASS) leaf area index (LAI) and Climate Research Unit (CRU) temperature and precipitation data, the temporal and spatial variation characteristics of vegetation in China’s arid and semiarid regions were analyzed from 1982 to 2017. The residual trend analysis method was employed to calculate the relative contributions of temperature, precipitation, and human activities to the variations in LAI. The results revealed that (1) vegetation was remarkably improved in most arid and semiarid areas, which was specifically pronounced in the summer; (2) in most of the study areas, vegetation was positively correlated with the temperature and precipitation, and temperature mainly affected the long-term trends in vegetation, while precipitation mainly affected its interannual variation; (3) spring and summer temperatures and summer precipitation substantially affected the vegetation growth during the same season, while in northern Xinjiang and central Inner Mongolia, a seasonal lag was observed in the response of vegetation to climate factors; and (4) vegetation changes were affected by human activities and climate change, and the dominant factors of vegetation change varied by regions. In places with improved vegetation, the contributions of climatic factors and human activities were 59% and 51%, respectively. Moreover, in areas with degraded vegetation, the contribution of climatic forcing was −51%, whereas that of human activities was −49%. Generally, the vegetation in northern China's arid and semiarid areas has experienced notable changes in recent decades, resulting from the combined effects of climate change and human activities.