Abstract: In order to understand the impact of water-salt on spatial development of cotton roots under mulch drip irrigation,144 root samples were collected in the flower and boll stage for two different treatments irrigated with saline groundwater and fresh surface water using same irrigation system. Screening out cotton roots from the soil with 1 mm soil sieve and hand-picking，removing dead roots, scanning into a tif format images after washing, then the root length density was obtained through DT-SCAN software calculation. The results show that the distribution area of roots under mulch drip irrigation with saline water is larger and total root length density (31.69 mm/cm3) is bigger than that with fresh water. The depth of root for saline water irrigation is much greater than that with fresh water because of salt stress. Root distribution is mainly affected by salt stress under saline water irrigation; however, it is affected by water stress under fresh water irrigation. Water stress plays a leading role in root development 40 cm away from the emitter at the horizontal direction. When the volumetric water content of soil is greater than 20 % and the electric conductivity is smaller than 2000 μs/cm, the root can grow and develop normally.