Using tree-ring width Chronology of Picea Crassifolia of the middle region of the Qilian Mountains, we studied the relationship between tree-ring and climate factors, and reconstructed spring precipitation of the middle region of the Qilian Mountains since A.D.1770.Well replicated tree-ring increment cores for Picea Crassifolia were sampled from the middle region of the Qilian Mountains.After carefully cross-dating and measurement of each core, standard chronology was developed according to chronology building procedure.The correlation between the tree-ring index and the mean precipitation and mean air temperature of Sunan station, Qilian station and Yeniugou station was analyzed.The results indicate that there is positive correlation between tree-ring index and spring precipitation.The correlation coefficient R is 0.505.The tree-ring width is limited by spring precipitation, which is beneficial to tree growth.A reasonable interpretation is that when the rainy season has not arrived yet and precipitation is deficient in spring, the soil water becomes the limiting factor to tree-ring growth.Narrow tree-rings are produced when the limited precipitation leads to shortage of soil water.The spring precipitation benefits the tree growth.Associating the study research in Daqingshan Mountain by Liu Yu, et al., we take a conclusion that the tree growth in arid and hemi-arid region is limited by spring precipitation.According to the tree-ring data, we reconstruct spring precipitation for the period A.D.1770 to 1995.Cross-verification testing of the reconstruction produces highly significant sign test and product mean and high positive reduction of error statistics.These prove that reconstruction is reliable.The reconstructed spring precipitation shows four humid and dry periods in the low frequency variations of 11 years moving average since A.D.1770.Humid periods are 1776-1795 A.D., 1819-1857 A.D., 1888-1906 A.D., 1943-1991 A.D..Dry periods are 1796-1818 A.D., 1858-1887 A.D., 1907-1942 A.D., 1992-1995 A.D..The reconstructed spring precipitation indicateds that the middle region of Qilian Mountains was mainly in a humid state since A.D.1770.Seen from the large scale for the last 230 years, there is no obvious humid or dry tendency for spring precipitation, but take on an alternative form.After analyzing periodic characteristics, we find spring precipitation since A.D.1770 has 69 years periods and 21 years periods.Contrasting to the climate change of neighboring area, we find that reconstructed dry periods correspond well with those of the Hengduan Mountains and Huashan Region.They also correspond with the drought disasters occurred in northwest of China.Suggesting that the spring precipitation, especially the abnormal dry change has large region representative, at least records dry humid change of north-west of China and it also indicates that the reconstruction is reliable.The period of the 1900s-1940s which shows low value for spring precipitation are corresponds well with the period of the east region of China and HuaBei Region and HuaiHe River.The period of the 1940s-1960s with high value is similar with that in the change tendency in most of the regions of China.So we take a conclusion that the spring precipitation probably has a closely relationship with the climate change of North Hemisphere or larger scale.