Photosynthesis coupled with transpiration determines water use efficiency (WUE) at leaf level, and the responses of WUE controlled by gas exchanges through stomata to environment were the basis of carbon and water cycle in ecosystem. In this paper, by using Li-6400 Portable Photosynthesis System (LI-COR), WUE at leaf level was analyzed under controlled photosynthetic photons flux density (PPFD) and CO2 concentration conditions across 9 plant species including maize (Zea mays), sorghum (Sorghum vulgare), millet (Setaria italica), soybean (Glycine max), peanut (Arachis phyogaea), sweet potato (Ipomoea batatas), rice (Oryza sativa), masson pine (Pinus massoniana) and Schima superba. We developed a new model to estimate the water use efficiency in response to the combined effects of light and CO2 concentration. Validation by our measured data showed that this model could simulate the changes of water use efficiency very good under combined effect of light and CO2 concentration. And it can estimate contribution of photosynthesis increase and transpiration decline on water use efficiency, owing to the CO2 concentration rising. Great differences in water use efficiency occurred in these different plant species under various CO2 concentration levels. We concluded that plants should be separated into C3 plants and C4 plants, farther, C3 plants should be separated into herbaceous plants and woody plants if based on water use efficiency at regional scale. Our separation criteria would do great help in the evaportranspiration modeling of terrestrial ecosystem carbon and water balance.