The evapotranspiration (ET) in mountainous watersheds affects the security of downstream water supply and ecological safety. Continuous time-varying ET cannot be obtained by traditional methods such as remote sensing and ground observations. In this study, a simple biosphere model 2 (SiB2) was parameterized in a typical mountainous area, the upper Minjiang River, using field observations and remote sensing data. The performance of SiB2 was then assessed by comparing it with direct measurements of the evaporation dishes. The results showed that (1) at the daily scale, the simulated ET was smaller than the measured ET. At the monthly scale, the relative errors between the simulated and measured values ranged from 1.48% to 20.72%. The relative error between the simulated and measured values of the total annual ET is 6.99%; (2) the ET of Minjiang River was characterized by a "bimodal" variation, with lower daily ET from November to February (non-growing season) and higher daily ET from March to August (growing season); (3) the ET of Minjiang River showed the spatial characteristics of low in the northwest and high in the southeast, with a high-value center located in Dujiangyan City. In summary, SiB2 is suitable for simulating ET in mountainous watersheds with spatial and temporal continuity. This study can contribute to the enhancement of water resources use planning and basin management in the upper Minjiang River.