Urban green spaces largely affect energy and mass exchange in urban ecosystem, and they can decrease temperature, increase humidity, absorb carbon dioxide and release oxygen. It is significant to quantitatively evaluate the impact of urban green space on urban environment. Base on the surface energy and CO2 flux measurements in Haidian Park, Beijing, the water, heat and CO2 exchange was analyzed in this paper. The results show that most of incident solar radiation is changed into latent heat flux (LE) during growing season, while it is mainly changed into sensible heat flux(H) during non-growing season. The ratio of H and LE decreases from spring to summer, with one-thirds in summer and several times in winter. LE is greater in the central park than that on the fringe of the park. The most important environmental factors controlling LE are net radiation, relative humidity and atmospheric temperature. The energy imbalance phenomenon is found in this study, with energy balance ratio ranging from 52% to 83%. It is also showed that the diurnal average of CO2 flux is negative. Urban green space is the sink of carbon dioxide during growing season.