The Taiga forest in the Altai Mountains of Xinjiang is the only distribution area of the West Siberian Taiga in China. Due to its location in a high-latitude inland region, relatively simple structure, and a large proportion of over-mature stands, its ecosystem functions are highly sensitive to environmental changes. This study investigated the effects of environmental factors and stand characteristics on Taiga forest biomass and productivity through forest plots established along environmental gradients in the Altai Mountains. The results showed that from northwest to southeast, tree biomass of forest decreased and then increased, while productivity significantly decreased. Water availability is the key environmental factor affecting both biomass and productivity. Increased precipitation not only significantly promoted stand biomass and productivity but also indirectly enhanced biomass accumulation by facilitating stand height growth. In contrast, heat conditions represented by potential evapotranspiration exhibited significant negative effects on stand biomass. Meanwhile, stand biotic factors primarily affected productivity indirectly through biomass mediation, and productivity exhibited a declining trend with stand age. Under intensifying climate change in high-latitude regions, aging Taiga forests may experience more pronounced ecosystem function degradation, necessitating heightened research attention.