Abstract: This study examined the physiological mechanisms underlying the rapid expansion of Veratrum oxysepalum Turcz. and its ecological effects on the alpine tundra zone of Changbai Mountain. Four fixed monitoring plots (2 m×2 m) were established at an elevation of 2 181–2 190 m in areas dominated by pure stands of Rhododendron aureum Georgi and surrounded by V. oxysepalum. Within the co-occurrence zone of R. aureum and V. oxysepalum, plots were classified into four invasion levels: non-invaded, lightly invaded, moderately invaded, and heavily invaded. From 2017 to 2020, changes in community composition were monitored, and physiological traits of the dominant species were quantified, including net photosynthetic rate (A), transpiration rate (E), water use efficiency (WUE), maximum quantum efficiency of PSⅡ (F_v/F_m), effective quantum yield of PSⅡ (φPSⅡ), and electron transport rate (ETR). Results showed that V. oxysepalum expanded rapidly across the tundra, with cover increasing from 0 to 55% within 4 years, and progressively replaced native R. aureum populations as the dominant species. Increasing V. oxysepalum cover was accompanied by a significant rise in community species richness, whereas R. aureum cover and plant density decreased by 70% and 78%, respectively. With increasing invasion intensity, V. oxysepalum showed significant increases in net photosynthetic rate and leaf area, together with strong positive correlations among photosynthetic parameters, indicating coordinated enhancement of photosynthetic performance. V. oxysepalum exhibited greater light energy conversion efficiency, stronger carbon assimilation capacity, and more efficient photosynthetic operation than R. aureum. Although R. aureum maintained higher WUE than V. oxysepalum, WUE increased further under stronger of invasion pressure and was significantly negatively correlated with net photosynthetic rate, suggesting a physiological trade-off in which carbon assimilation was reduced to maintain water balance under stress. At the end of the growing season, V. oxysepalum also significantly inhibited the ETR of R. aureum, further weakening photosynthetic capacity. This negative physiological feedback limited the capacity of R. aureum to tolerate invasion pressure, accelerated niche contraction, and contributed to the loss of competitive dominance. Overall, the strong photosynthetic capacity of V. oxysepalum represents a key driver of its rapid expansion and competitive displacement of R. aureum in the Changbai Mountain alpine tundra. Ecological restoration and management strategies, including artificial suppression, biological antagonism, and microhabitat restoration, are therefore needed to mitigate potential degradation of tundra vegetation caused by V. oxysepalum expansion.