Abstract: A tree-ring chronology has been built up based on the tree-ring samples from the Qilian Mountains. Response analysis showed that there is significant relationship between the tree-ring width and the June to July runoff of the Datong River watershed. Based on the analysis, a transfer function was designed to reconstruct the total runoff from June to July of the Datong River during the period 1525-2009. There are 5 wet and 5 dry periods in the reconstructed runoff history. The wet periods are 1530-1555,1576-1586,1733-1760,1776-1786,1855-1912, and the dry periods 670-1680,1691-1730,1814-1836,1926-1942,1964-1978 respetively. The reconstructed series also displayed that the standard deviations in the dry periods are higher than the wet periods, and the extreme dry and wet periods have the highest standard deviation. The reconstructed June-July runoff series shows significant periodicities around 2 (2.04~2.3),50 (50~57),100, 133 and 200years.