Arceuthobium sichuanense is a hemiparasitic angiosperm that infects Qinghai spruce (Picea crassifolia Kom.) in Qinghai province, China, and causes severe damage to spruce forests in Qinghai-Tibet Plateau. In this study, the impact of A. sichuanense infection on mature and young trees of Qinghai spruce was evaluated by examining needle and current-year shoot morphology, needle water and nitrogen-use efficiency (NUE) and needle nitrogen concentration. The most apparent effect of A. sichuanense infection was a significant reduction in both needle size distal to infection and current-year shoot length in the infected branches (p < 0.001). Per cent reductions in needle and current-year shoot length were similar between mature and young trees (58.9 vs. 56.3%; 59.7 vs. 62.9%). There was a high degree of correlation in foliar δ15N values between the dwarf mistletoe and its host trees (R2 = 0.9017, p < 0.001), while the foliar δ13C values of A. sichuanense were similar to those of infected mature and young spruce trees. The dwarf mistletoe infection also resulted in a significant decrease in host needle N concentration and δ13C values (p < 0.001). The per cent reduction in needle N concentration in young trees was nearly twice as much as that in mature trees (20.49 vs. 11.54%), while the per cent reduction in needle δ13C values was similar between young and mature trees (−0.98 vs.−1.1‰). The NUE in mature trees was not affected by A. sichuanense infection, but the NUE in young trees was increased by the infection.