Leaf stable isotope compositions (δ¹³C and δ¹⁵N) are influenced by various abiotic and biotic factors. Qinghai spruce (Picea crassifolia Kom.) as one of the dominant tree species in Qilian Mountains plays a key role in the ecological stability of arid region in the northwest of China. However, our knowledge of the relative importance of multiple factors on leaf δ¹³C and δ¹⁵N remains incomplete. In this work, we investigated the relationships of δ¹³C and δ¹⁵N to leaf age, tree age and leaf nutrients to examine the patterns and controls of leaf δ¹³C and δ¹⁵N variation of Picea crassifolia. Results showed that ¹³C and ¹⁵N of current-year leaves were more enriched than older ones at each tree age level. There was no significant difference in leaf δ¹³C values among trees of different ages, while juvenile trees (<50 years old) were ¹⁵N depleted compared to middle-aged trees (50–100 years old) at each leaf age level except for 1-year-old leaves. Meanwhile, relative importance analysis has demonstrated that leaf age was one of the most important indicators for leaf δ¹³C and δ¹⁵N. Moreover, leaf N concentrations played a dominant role in the variations of δ¹³C and δ¹⁵N. Above all, these results provide valuable information on the eco-physiological responses of P. crassifolia in arid and semi-arid regions.