Populus euphratica Olve. is a dominant tree species in Ejinaa Oasis of the lower reaches of the Helhe River, Inner Mongolia, China. In recent years, the population of this species has shown signs of degradation that are suggested to be probably associated with the decrease of surface water. In this study, the history of river runoff In this region was examined with a dendrochronological study of P. euphratica from four sites. it is found that tree-ring widths in the all sites have negative correlation with runoffs of all months at the Zhengyixia hydrological station. Principal component (PC) analysis of the tree-ring chronologies shows that the first PC (PC1) accounts for 49.98% of the total variances. The first PC is found to have a significantly negative correlation with runoffs In March and April (-0.52 and- 0.43, respectively, P＜0.05). This negative correlation might be related to varlatlons In the depth of underground water.