[Objective] The changing trend of water-holding function in a subalpine area of Western Sichuan Province was analyzed in order to provide a scientific basis for defining the dynamic characteristics of water conservation in this area. [Methods] Taking the Zagunao River basin in the upper reaches of the Minjiang River as an example, the variation characteristics of water conservation capacity of the Zagunao River basin were studied based on precipitation, temperature, water, and sediment data collected at the outlet of the Zagunao River basin and by using the principle of water balance. [Results] ① Runoff was significantly reduced over time, with an average annual decrease of 2.83 mm (i.e., water-holding capacity increased by 28.3 t/ha annually), especially around year 2000 (when the ecological protection and ecological restoration project started) to 2020 when the average annual decrease was 3.12 mm (i.e., water-holding capacity increased by 31.2 t/ha annually); ② Sand content in the runoff also showed a significant decreasing trend, indicating that runoff in the area was decreasing while soil conservation capacity was increasing; ③ There was no significant fluctuation trend in precipitation over the study period, indicating that the observed runoff decrease was not caused by precipitation changes. [Conclusion] Ecological engineering, such as ecological protection and restoration, plays an obvious role in promoting water conservation.