Abstract: The changes of photosynthetic rate (Pn), transpiration rate (Tr) and water use efficiency (WUE) of maize seedlings under soil progressive drought were studied. The results show that photosynthetic rate and transpiration rate increase a little with the declining of soil water content during the early drought days. When soil moisture is 90% of field water capacity (FWC),they both reach the maximum. Then they begin to decrease with unceasing drought. The rate of decreasing is slow at first, but when soil moisture is below 70% of FWC, it decreases linearly with decreasing soil water.The changes are caused synthetically or alternately by stomatal and non-stomatal factors. Leaf water potential-0.8MPa is an important threshold in enhancing photosynthetic rate and water use efficiency.