Evapotranspiration (ET) in the terrestrial ecosystem actually includes canopy transpiration (E_c), soil evaporation (E_s), and interception evaporation (E_int). Partitioning of ET is an important basis in order to accurately assess biomass production and estimate water use efficiency in the terrestrial ecosystem. Actually, E_c is the desired component within the water cycling, which being used to enhance plant productivity. Up to now, it is still difficult to accurately partition ET through observation methods in the long period. Instead, partitioning ET with models is effective method in a long period. However, most of them roughly partition ET into E_c and E_s, ignoring E_int. This partition way could lead to deviation in E_c and E_s simulation. In this study, a modified Shuttleworth-Wallace model was used to partition ET into E_c, E_s, and E_int in a forest ecosystem. Monte Carlo, an approach of random parameterization, was performed to optimize the key parameters in the functions estimating soil surface resistance and canopy stomatal resistance in the model. Based on the modified model, we simulated the evapotranspiration and its components in the planted coniferous forest ecosystem at the Qianyanzhou site combining the eddy covariance measurement data (including sensible/latent heat flux and CO₂ flux) with routine meteorological data(including air temperature/humidity, CO₂ concentration, wind speed, net radiation, soil temperature/water content/heat flux and precipitation) in 2011. The results showed that the simulation amount of ET agreed well with the measurement data at 30 minutes temporal scale not matter on sunny days or rainy days. At the same temporal scale, the index of agreement, mean error, and root mean square error between simulation value and measurement data in whole year were 0.73, 0.21 mmol m^-2 s^-1, and 1.55 mmol m^-2 s^-1, respectively. The proportion of ET in annual accumulative precipitation was more than 80%, which meant that the amount of ET contributing to the water outputs is the most in this ecosystem. The proportion of canopy transpiration in ET was about 85%, which suggested that water use efficiency in the forest ecosystem might be quite high in 2011. The seasonal variation of the transpiration was obvious. The environmental factors, vapor pressure deficit, air temperature, and leaf area index controlled transpiration. Transpiration correlated positively with the three factors. Soil evaporation was 5% of the ET and changed little with season. Interception evaporation accounted for 10% of the ET and showed seasonal dynamic. This term correlated positively with precipitation and negatively with rainfall storm events. This result indicated that the vegetation canopy couldn't effectively intercept the heavy rainfall. The modified Shuttleworth-Wallace model is a powerful tool for studying the hydrological processes in terrestrial ecosystem with the advantages of a few parameters, high temporal resolution, and a good performance in simulating the characteristics of evapotranspiration and its components.