Abstract: The upper reaches of Minjiang River is not only an important part of the ecological barrier in the upper reaches of the Yangtze River, but also a vital water source lifeline for the Chengdu plain. InVEST model and shadow engineering method were used to assess the ecosystem water retention and its value in the upper reaches of Minjiang River, and this work had important significance for water conservation planning and establishment of payment for ecosystem service mechanism. The results showed that the average annual water retention amount of sub watersheds in the upper reaches of Minjiang River was 219.36 mm, the total water retention amount was 49.19 billion m ³ and the water retention value was 4.083 billion Yuan. Water retention amount and its value in our study area displayed some obvious spatial variation with high value areas mainly distributed in south central Wenchuan County and low value areas relatively concentrated in the regions of Songpan and Maoxian County. Due to the limitation of the evaluation method, the relevant findings of this watershed were obviously different and had poor comparability. However, it is sure that the upper reaches of the Minjiang River has huge water retention value. The quantitative assessment results and mapping can directly reflected the spatial variation in water retention and its value. It had strong applied value in the establishment of payment for ecosystem service mechanism and compensation efficiency analysis etc.