The Bay an Har Mountains are located in the water divide of upstream regions of the Yangtze and Yellow Rivers, China; their ecological environment and climate change has important influences on ecological security of the downstream areas. Meanwhile, the Bayan Har Mountains are situated on the eastern margin of Qinghai-Tibetan Plateau and the climate transition zone from the alpine climate zone to the eastern subtropical monsoon zone. Thus, the climate variability in the study area is sensitive to the global climate change. However, the history and mechanism of climate variability in this region is poorly understood due to the scarcity of long-term instrumental data. In this paper, a 486-yr tree-ring width chronology of Picea crassifolia is established with 57 cores from 29 trees. The sampling site (YBG) is located on the eastern Bayan Har Mountains (32°37'N, 101°34'E; 3553 m a.s.l.). The Subsample Signal Strength (SSS) of the standard tree-ring chronology was larger than 0.85 during the period from 1637 A.D. to 2012 A.D. The limited factor of the tree-ring growth in study area was considered as mean summer (June,July and August) temperature in the growth year,with a correlation coefficient of 0.59 (p<0.01). Accordingly, we reconstructed summer mean temperature variability over the past 376 years. The equation of reconstruction was stable,sharing 34% of the total variance of the observed data over the common period. The turning points of the accumulative anomaly series are illustrated that the reconstructed summer temperatures over the past 376 years included two cold periods (1724~ 1789A.D. and 1864~ 1994A.D.)and three warm periods (1637 ~ 1723 A.D., 1790 ?1863 A.D. and 1995 ~ 2012 A.D.). The Ensemble Empirical Mode Decomposition (EEMD) results show that the decadal and multi-decadal summer temperature variability is related to the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO). This indicates that the reconstructed summer temperature may be affected by the sea surface temperature (SST) over North Pacific and North Atlantic. The climate model simulation is needed to reveal its mechanism.