Studies on cambial phenology and wood formation dynamics (xylogenesis) can provide mechanistic understandings that how climate influence wood production and carbon sequestration at cellular level. The Qilian Mountains is an important ecological security barrier in northwest and even the whole China. However, there is very little in-depth information on the physiological responses of trees to changing climate, especially at individual level. Here, we analyzed phenological traits related to the observed cambial phenology and xylogenesis over two extreme hydroclimatic years from the central Qilian Mountains, northwestern China. The cambial phenology and xylogenesis were assessed biweekly from microcores of five Qilian juniper (Juniperus przewalskii) trees. Results showed that the onset of cell wall-thickening and the first mature cell occurred on June 26 and July 24 in 2012 growing season, 5 and 28 days later than those in 2013 growing season, respectively. In 2012, the end of cell enlargement, cell wall thickening, and cell lignification phase occurred on July 16, August 9, and September 8, respectively, which were 7 days, 28 days and 24 days later than the end of each developing phase in 2013 growing season. In 2012, the maximum rate of wood production was 0.33 cells/d, higher than that in 2013 growing season (0.28 cells/d). Accordingly, a total of 21 cells were formed in 2012 growing season, four cells higher than that in 2013. By comparison with meteorological data recorded at a nearby meteorological station, we found that the cambial phenology and xylogenesis of Qilian juniper were sensitive to changing climate. The onset of xylogenesis in the warm year (2013) was significantly earlier than that in the colder year (2012), indicated that the onset of the growing season was mainly related to temperature. In contrast, warm conditions in 2013 led to an intensification of regional drought in spring and early summer, resulted into a 24 days earlier at the end of the growing season than that in 2012. Our results indicated that strong shifts in water availability during the growing season were crucial to the growth rate and final wood production. Our findings suggested that while warming promoted an early growing season, it also had a strong influence on tree growth and productivity in cold and arid regions due to the decrease in moisture availability. Our study provides new evidence on the malleable nature of tree growth in response to climatic variations in cold and arid region.