To quantitatively investigate the water conservation function of the natural spruce forest at the watershed scale in the central Tianshan Mountains, the Urumqi River Basin was chosen as a study area. The InVEST model was used to investigate the capacity for water conservation of the spruce forest and other land use types, as well as quantitatively analyze water conservation changes of the spruce forest under the conditions of different distribution areas and terrain factors. The results showed that: (1) The InVEST model could adequately assess water conservation at the watershed scale. The InVEST model simulation showed that the total water conservation of the spruce forest in the Urumqi River Basin was 4.93×10⁶ m³, which accounted for 20.46% of the total water conservation in the study area (2.41×10⁷ m³). In addition, the average water conservation depth of forest area was 54.25 mm. (2) The contribution rate of water conservation in the slope, aspect, and elevation was consistent with the spatial distribution pattern of the spruce forest. At first, per hectare water conservation increased with altitude and then decreased, and it decreased with an increase in slope. The water conservation ability in the shade slope was greatest. (3) The spruce forest showed the greatest capacity, which reached 544.78 m³/hm². The total, average depth and unit area of water conservation exhibited an increasing trend with increasing spruce forest coverage. (4) The contribution rate of water conservation in the spruce forest was closely related to forest area. Total water conservation in study area and spruce area increased to 0.437×10⁶ m³ and 0.522×10⁶ m³ with a 1% increase of spruce forest area, respectively. Therefore, the forest plays a vital role in maintaining sustainable and stable water conservation and ecological service functions, which necessitate effective measures to strengthen the protection and management of spruce forests.