Background: Tree-line areas exhibit significant changes in response to climate change, including upward migration. Lower tree line dynamics are rarely studied, but as unique features in arid and semi-arid areas, they may influence forest distribution. Here, 8 lower tree-line plots in Picea crassifolia Kom. (Qinghai spruce) forest in the arid and semi-arid Qilian Mountains of northwestern China were used to determine changes in tree line location, landscape pattern, and relationships with meteorological factors during 1968-2018. Results: The results showed that the lower tree line descended by an average of 9.82 m during 1968 to 2018, and exhibited almost no change after 2008. The change in pattern was mainly related to the age of trees in which small trees were dense and clustered, and large trees became more scattered and evenly distributed. Tree regeneration rates changes in the lower tree line were highly positively correlated with temperature, and also positively related to annual precipitation. Conclusions: In the past 50 years, the lower tree line in arid areas exhibited a downward trend but it is unclear whether the downward trend of the lower tree line will stabilize or even reverse due to the weakening of climate warming degree.