The leaf area index (LAI) controls a variety of biophysical and physiological processes of vegetation canopy, such as photosynthesis, respiration, transpiration, carbon cycle, precipitation interception and energy exchange, thus it is an important parameter for characterizing crop growth as well as studying terrestrial ecosystems. In recent years, with the continuous increase of greenhouse gas emissions, the whole globe has gone through an intensifying warming trend. Up to now, a large number of agricultural experiments, such as Free Air CO₂ Enrichment, (FACE) and Open-Top-Chamber(OTC), show that elevated CO₂ concentration will cause reduced plant transpiration rate, strengthened stomatal resistance and a significant increase in water use efficiency. Agriculture, which is quite sensitive to climate change, will be significantly affected under the context of global warming. In this paper, the Decision Support System for Agrotechnology Transfer (DSSAT) developed by the U.S. Department of Agriculture is used to simulate the growth process of winter wheat under three different CO₂ concentrations combined with water deficiency and adequacy status. The observed experimental data over 2007-2008 is from Gucheng Ecology and Agricultural Experiment Station, Chinese Academy of Meteorological Sciences. Using DSSAT, the impacts of environmental CO₂ concentration change on crop LAI are analyzed. The results indicate that elevated CO₂ concentration will strengthen the leaf area index growth, with more significant positive effects on LAI under drought condition (water supply:150mm) than under the wet condition (water supply: 620mm), At current CO₂ level, crop LAI under water deficiency condition is significantly less than that under wet condition; when the CO₂ concentration is increased by 20%, the difference of LAI under dry and wet conditions declined slightly; when the CO₂ concentration is doubled, the maximum of LAI under wet conditions increased by 0.31 and LAI under the water deficiency increased by 0.82, which leads to the conclusion that crop LAI under water deficit condition increased several times at doubled CO₂ level. Contrast to both concentrations of water deficit and sufficient, when CO₂ concentration doubled, it is significantly alleviate the adverse effects of leaf area index in the water deficit condition. Over time series, the change of LAI is very small when CO₂ concentration changed before returning green stage (about 130 days after planting). However, the influences of CO₂ concentration change on crop LAI become significant after returning green stage. Also, there is another study which shows that the combination of elevated CO₂ concentration and drought stress can increase the air and canopy temperature, offsetting the fertilizer effect of elevated CO₂ concentration. The study facilitates the analysis of the impacts of CO₂ concentration changes on crop growth process, which can provide the basis for the agricultural water management and propose a model approach for estimating LAI.