In China and East Asia, characteristics of energy exchange in the Tibetan Plateau and its surrounding areas are important factors for promoting the processes of weather change and climate change. The energy budget cannot be ignored because of the unique thermal structure of the atmospheric boundary layer of the Qinghai-Tibet Plateau and its surrounding valleys, as well as of the transition zone between the Qinghai-Tibet Plateau and Sichuan Basin. However, because of the lack of long-term observational data, the characteristics of energy balance in these areas are unclear. In this study, we installed an eddy covariance system over a coniferous and broad-leaved mixed forest in Jiuzhaigou Valley, which is a typical subalpine valley in the transitional zone extending from the northeastern margin of the Qinghai-Tibet Plateau to the Sichuan Basin. The turbulent energy fluxes and available energy over this forest were continuously measured for two years, from 2014 to 2015; the characteristics of energy balance were analyzed and energy balance closure was discussed. The results showed that the diurnal and seasonal variations in energy balance terms were similar to those in net radiation. When the pre-growth, growth, and post-growth periods were compared, a 2-3 hour lag was observed in the peak time of each energy balance term during the frozen-soil period. Moreover, the values of each term varied in different seasons. Sensible heat flux dominated the energy terms in the pre-growth and frozen-soil periods, while available energy was mainly consumed by latent heat flux in the growth and post-growth periods. In the growth period of 2014 and 2015, the ratio of latent heat flux to net radiation was 0.69 and 0.75, respectively, and these values are dramatically higher than those in other parts of the Qinghai-Tibetan Plateau. The energy budget was unbalanced in the two years, and the energy balance closure was 0.75 and 0.71, respectively. These results are essential for a better understanding of the energy cycling mechanism and dynamics from the plateau to the basin, as well as in the mountainous areas of the Tibetan Plateau.