Mountain vegetation exhibits unique growth strategies along elevation gradients to adapt to climatic constraints. However, the mechanisms by which temporal allocation of photosynthetic phases and climate change drive elevational differentiation in carbon sequestration dynamics remain poorly quantified. This study adopted a phenology-based method to divide the photosynthetic growing season into distinct stages, revealing vegetation carbon allocation and its elevation-dependent patterns at a finer temporal resolution. In the Qilian Mountains (QLMs), although the maturity period was only 1.2 times longer than the greening period, it contributed fivefold greater gross primary productivity (GPP) during growing season, highlighting its pivotal role in GPP dynamics. Notably, GPP during both the photosynthetic growing season and maturity period exhibited greater relative rates of change at high elevations (>
3500 m) than at lower elevations (2500-3500 m). Concurrently, vegetation at higher elevations displayed greater temperature sensitivity. For every 1000 m increase in elevation, the maturity period lengthened by 3.4%, while the greening and senescence periods shortened, maximizing carbon sequestration under colder conditions. Analysis through boosted regression trees and partial least squares regression revealed a dual-control mechanism governing GPP through hydrothermal conditions and growth-stage duration. Temperature dominated GPP during growing season and maturity period, whereas growth-stage duration exerted predominant influence on greening and senescence periods. The observed trend of vegetation homogenization along the elevation gradient in the QLMs could reduce ecosystem resilience and carbon sequestration capacity. Continued monitoring and research are crucial for understanding these impacts and guiding ecosystem management in high-altitude regions.