Using brackish and saline water is one of the most effective ways to solve the problem of water resource shortage. To rationally exploit and make use of brackish and saline water resources, the effects of drip irrigation underneath a plastic film with different salinities of irrigation water on soil salt content and cotton growth were studied at the Aksu National Field Research Station of Agro-ecosystem in Xinjiang from 2008 to 2009. The results showed that the soil salt increases before the first irrigation of the growing period, then generally decreases during the irrigation period, and increases after the final irrigation of the growing period and before winter irrigation. In extremely arid areas, the leaching effect of rainfall is weakened. When cotton is irrigated with saline water of salinity greater than 2·24 g l−1, the salt is accumulated at the main cotton root zone. The effect of salinity of irrigation water on cotton growth is a gradual process and is highlighted during the boll-opening period. Leaf area index, root area index, root length density, and root weight density all decrease with an increase of salinity of irrigation water and soil salt content in the root zone. The root weight density and root length density of treatments with brackish or saline water are far less than those of treatment with fresh water. After sowing, with the increase of time, the leaf area index changes from small to large, reaches its peak at the flowering-bolling stage, and then reverts from large to small; however, root area index, average cotton root length density, and root weight density of the 0- to 50-cm soil layer continuously increase throughout the entire process. During the boll-opening period, the vitality of the cotton root remains strong. Copyright © 2012 John Wiley & Sons, Ltd.