Human proximity to water and its dynamics are important to understanding the socio-hydrological dilemma between using floodplains and avoiding flood risk. However, previous studies have not distinguished between the water proximity of built-up land in floodplains (BLF) and the water proximity of built-up land outside floodplains (BLOF). This article provides a new and spatiotemporal perspective of the BLF water proximities for understanding the changing flood risk in China. The results show that China’s BLFs had an average water proximity of 5.41 km in 2014. Most of the BLFs (62%, 25.88×10³ km²) were located within 3 km of waterbodies. From 1990 to 2014, China’s BLFs increased rapidly by 81% from 23.06×10³ km² to 41.74×10³ km², of which a large portion (57%) was concentrated in water surroundings (≤3 km), shortening the distance between BLFs and waterbodies by 169 m. The BLF growth concentrated in water surroundings even in areas where BLFs have an overall increasing distance from waterbodies. Both the increases in the BLFs and their proximity to waterbodies can increase flood exposure and exacerbate flood risk. The scientific community and policymakers should pay attention not only to the volume of BLF growth, but also its spatial relationship with waterbodies.