The forest edge usually formed by felling trees and forest fire is a typical ecotone with high biodiversity and rapid changing light. It is of important significance to study on how the different functional groups to use the solar energy use and water so as to understand the physiological ecological basis of forest regeneration, forest community dynamics and evolution at individual level. Quercus mongolica is a common sun plant in deciduous broad-leaved forests and broad-leaved Pinus korinsis forests in Changbaishan Mountains. Based on the field experiment on Q. mongolica trees near the edge of the natural secondary white birch forest, the following results could be achieved: ① The effect of forest canopy on forest edge light varies according to the distance from the forest edge. The light in the forest edge and near the edge changes enormously. The total amount of daily PAR of the forest edge is about 50% of that outside the forest, while the total amount of daily PAR within the forest is about 5% of that outside the forest. Moreover, the air temperature, land temperature and humidity across the forest edge change significantly. ② Q. mongolica tree has a single peak curve in the daily process of photosynthesis, and net photosynthetic rate (Pn) reaches a peak (about 20μmol CO₂•m^－2•s^－1) at 10：00～12:00. No significant midday photosynthetic depression is observed in sunny day in summer, and the reason should be further studied. Meanwhile, the net photosynthetic rate and total amount of daily photosynthesis of Q. mongolica tree are greater (or far greater) than those in forest edge and within the forest. This indicates that Q. mongolica tree grows the most rapidly under strong light outside the forest. In addition, this deciduous tree has so high utilization efficiency to low light within the forest that Q. mongolica trees can normally grow within the forest. ③The daily process of transpiration rate of Q. mongolica trees under different light environment is single peaked with a maximum of about 8 mmol H₂Ｏ•m^－2•s^－1 outside the forest. Although Mongolia oak has fairly high transpiration rate, daily transpiration amount and water utilization efficiency, yet as compared with low light environment outside the forest, this increasing trend of water utilization has slightly reduced. So, this could be thought as a kind of acclimation to strong light environment outside the forest. ④ The stomatal conductance of Mongolia oak leaves increases along with the increase of light. It is beneficial to increase net photosynthesis rate and transpiration rate of plant under strong light conditions, and beneficial to rapid growth of plant under strong light conditions. In addition, the leaf intercellular CO2 concentration of Q. mongolica trees has a trend of increasing from morning till night. Along with the increase of the total amount of solar radiation under different light regimes, the leaf intercellular CO₂ concentration of Mongolia oak has a trend of decrease. Under low light conditions, the increase of leaf intercellular CO2 concentration is beneficial for plants in CO₂ fixation, increases solar energy utilization and improves water utilization efficiency.