Climate warming influences the physiological and ecological traits of trees across different elevations, as well as the CO2 fertilization effect on tree growth. To investigate the impacts of climate warming and rising CO2 concentrations on tree growth, we collected tree-ring samples of Qinghai spruce (Picea crassifolia) along an elevational gradient on the northeastern Tibetan Plateau. By analyzing tree-ring stable isotopes, we quantified interannual variations in intrinsic water use efficiency (iWUE) and assessed the effects of climatic factors and rising iWUE on tree growth across elevations. Our findings indicate that: (1) Since 1965, radial growth of Qinghai spruce has increased at all elevations, while iWUE has risen continuously, and tree-ring delta 18O and Delta 13C have shown declining trends over the same period; (2) The combined effects of rising atmospheric CO2 concentration and aridification have driven a sustained increase in iWUE; and iWUE is negatively correlated with tree growth (detrended BAI), particularly at high elevations; (3) Tree growth (detrended BAI) at both high and low elevations is primarily constrained by water availability. Our results further suggest that tree growth at high elevations may be substantially influenced by intrinsic ontogenetic factors such as tree age. After detrending to remove age-related trends, the growth enhancement from increased iWUE is weak at high elevations. This study provides important insights for developing elevation-specific forest management and conservation strategies in arid regions.