Tree-ring standardized chronologies are developed by 78 cores collected from the eastern and western Helan Mountain. Statistical analysis shows that both the STD and RES chronologies correlate negatively with the temperature of different periods of early half year,especially with January to August mean (JA) temperature,which means that JA temperature is one of the predominant limiting factors of tree growth in the Helan Mountain. Based on this analysis,we reconstructed JA temperature,and the explained variance is 43.3% (F=21.422,p<0.001). The comparatively high temperature periods in the reconstruction were: 1805?1818,1828-1857,1899-1907,1919-1931 and 1968-1995; and the comparatively low temperature periods happened in 1858-1872,1883-1895 and 1935-1953. Ten-year moving average curve shows three slow uplifting trends: 1766-1853,1862-1931 and 1944-1995. Each tem-perature increase was followed by a sudden temperature decrease about 10 years,that is to say,the JA temperature in the Helan Mountain is characterized by slow increase and sudden decrease. The 70-and 10.77-year periodicities detected in the temperature series correspond to the Gleissberg (80-year) and Schwabe (11-year) periodicities of solar activity respectively,the 2.11-2.62 years cycles are considered to be influenced by QBO (Quasi-Bie-nnial-Oscillation) and the local environmental change.