The northwest of the Beijing-Tianjin-Hebei (BTH) region is an important water conservation area and ecological barrier for Beijing and Tianjin. It is also an important regional support node for the "Beijing-Tianjin-Hebei coordinated development strategy". In order to reveal the spatial and temporal changes of groundwater level and its driving factors in northwest BTH region, 56 wells of groundwater level observation were selected from 1981 to 2015 in Zhangjiakou city as a typical region. Based on continuous observation data on hydrogeology, meteorology, groundwater resources exploitation and socioeconomic development, the spatial and temporal changes of groundwater level and its statistical analysis with the abovementioned data were performed in four geomorphic units of Bashang Plateau, Chaixuan Basin, Zhuohuai Basin and Yuyang Basin in the study area. Results showed that the groundwater level of the four geomorphic units in Zhangjiakou and districts and counties under its jurisdiction in general tended to decrease, and the decrease rate was gradually accelerated. In the past 35 years, the groundwater level of Bashang Plateau decreased by 3.59 m < 3.6 m in Yuyang Basin < 7.17 m in Zhuohuai Basin < 20.41 m in Chaixuan Basin. The changes of groundwater level in the four geomorphic units were not significantly correlated with natural factors such as precipitation, evaporation and temperature, but significant correlation between the total population and other socio-economic factors, including primary industry output value, common cultivated land area, effective irrigation area, total grain output, total vegetable output, total pork, beef and mutton output, secondary industry output value, tertiary industry output value and year-end total population. In the principal component analysis, the cumulative variance contribution rate of the first two principal components of the four geomorphic units was more than 80%. The first principal component was mainly socio-economic factors such as total vegetable production, total pork, beef and mutton production, year-end total population and output value of the primary, secondary and tertiary industries. All the first two principal components in the four geomorphic units can explain more than 66%, indicating that socio-economy was the main factor leading to the continuous decline of groundwater level in Zhangjiakou. The findings can provide new insights into the sustainable development of social economy and the rational utilization and allocation of regional water resources in the future, as well as water conservation and ecological function maintenance in the northwest of the BTH region.