Abstract: The investigated site in the Helan Mountains, is located on northwest margin of East Asian Summer Monsoon circulation, a semiarid region in North China. Three sets of tree-ring width residual chronology are used in this study, which were built using samples collected in south, middle and north of the mountains respectively. These three chronologic data were combined to form Helan chronology. In addition, meteorological data of three stations close to sampling sites were used, respectively in the east slope, on the top, and in the west slope of the mountains. The normalized data from these three meteorological stations were averaged to represent climatic situation of the Helan Mountains. Through linear regression analysis between the Helan chronology and the Helan meteorological data, the normalized precipitation of February-July from 1802 to 1997 was reconstructed and the explained variance was 46%. After 11-a moving standard deviation was calculated on the reconstructed precipitation series, three abrupt changes of standard deviation were found:one took place in 1908, increasing from 0.38 of the previous year to 0.56; the second was in 1942, increasing from 0.57 two years before to 0.70; and the third was in 1956, decreasing from 0.74 of the previous year to 0.63. Wavelet analysis showed that a dominant 11-a quasi-period occurred during 1908-1949, and that a 2-a quasi-period began to predominate in 1942, and then the 2-a quasi-period disappeared completely in 1959. There were significant 8-a transiting to 3-a periods from 1965 to 1997. The 11-a period probably corresponds to the sunspot activity and the 2-a period may correspond to "Quasi-biennial Oscillation". 8-to 3-a periods are similar to El Niño/Southern Oscillation periods. Thus, the precipitation variability might be high and the yearly precipitation became unstable in the Helan Mountains when the sunspot activity, Quasi-biennial Oscillation, or El Niño/La Niña activity were strengthened.