Precipitation isotope data were used to determine the origin of groundwater in the Badain Jaran Desert (BJD) in the study of Wu et al. (2017). Both precipitation and its isotope composition vary seasonally, so arithmetic averages of precipitation isotope values poorly represent the isotope composition of meteoric water. Their finding that the BJD groundwater is recharged by modern meteoric water from local areas including the southeastern adjacent mountains was based on arithmetic averaging. However, this conclusion is not supported by the corrected mean precipitation isotope values, which are weighted by the precipitation rate. Indeed, the available isotopic evidence shows that modern precipitation on the Qilian Mountains is more likely to be the main source of the groundwater and lake water in the BJD, as found by Chen et al. (2004).