The dioecious plant, Hippophae rhamnoides, is a pioneer species in community succession on the Qinghai-Tibet Plateau (QTP), plays great roles in various ecosystem services. However, the males and females of the species differ both in their morphology and physiology, resulting in a change in the ratio of male to female plants depending on the environment. To further explore the functional traits critical to this sex-based distinctive response in the alpine grassland, we have surveyed the sex ratios, measured their photosynthetic parameters, height, leaf area and biomass allocation. The results showed that (i) The males had higher Pn, light saturation point, apparent quantum efficiency, A_max and lower water-use efficiency (WUE), which exhibited higher utilization efficiency or tolerance to strong light, while the females indicated higher utilization efficiency for low light and water. And it showed sex-specific biomass allocation patterns. (ii) H. rhamnoides populations across the successional stages all showed a male-biased sexual allocation, which was closely related to sex-specific WUE, Pn, root biomass/total biomass and root–crown ratio. (iii) The leaf traits of H. rhamnoides changed from higher N_area, P_area and leaf mass per area in the early and late to lower in the middle, which meant they moved their growth strategy from resource rapid acquisition to conservation as the succession progressed. (iv) The increasing soil total phosphorus mostly contributed to regulating the sex bias of populations and variations of traits during the succession. The results are vital for the management of grassland degradation and restoration due to shrub encroachment on the QTP.