Forest structure and function are subject to the risk of tree growth declines related to external disturbances. However,few researches have been done to monitor long time scale tree growth declines and thus it is hard to assess the effect of extreme events on forest. In this study,we found that tree growth in the Three-River-Source National Park,Qumalai Prefecture was positively correlated with previous October mean temperature and May-June precipitation while negatively correlated with May to June mean temperature. We utilized dendrochronological methods to study the history of tree growth declines with the samples collected from Juniperus tibetica forest in Qumalai Prefecture. By analyzing the radial growth trajectories,we identified tree ring indexes of the three periods,including 1685-1695,1741-1749 and 1815-1824,showed a downward trend and two major events of intense tree growth decline happened in 1686-1697 and 1815-1827 in the past 372 years. Climate change resulted inLittle Ice Ageand Tambora eruption were most likely the reason for the two decline periods respectively and thus we concluded that rapid change of moisture and thermal conditions might play an essential role in tree growth. Since the 20th century the J. tibetica forest has been in a good condition for its growth though it was getting warm and dry in the context of global warming. Our study results will help to anticipate the forest health in response to climatic and environmental change in the future. Targeted protection of trees is of great importance for maintaining forest ecosystem services in the Three-River-Source National Park.