By using bidirectional sampling method with soil drill, the effects of different amounts of drip irrigation (2618, 2947, 3600 and 4265 m3 x hm(-2)) under mulching on the root distribution, aboveground growth, and yield of cotton was studied in field. The results indicated that irrigation amount affected the root and shoot growth significantly. In all irrigation treatments, cotton root was mainly distributed in mulched area, occupying 60.65%-73.45% of total root biomass, while only 39.35%-26.55% was distributed in bare area. Water stress increased rooting depth, root biomass, and the extent of lateral rooting. Significant differences were observed in the biological characteristics and the biomass accumulation and allocation of cotton plant among different irrigation treatments. Over-irrigation (4265 m3 x hm(-2)) increased plant height, width of inverse fourth leaf, and amounts of branch and bud, and thus, accelerated biomass accumulation rate. Over-irrigation also increased the root/shoot ratio and the proportion of biomass allocated to vegetative organs, but increased the fruit abscission rate and therefore reduced the economic yield. It was suggested that both excessive soil moisture content and water stress could affect the biomass accumulation and allocation in different cotton organs and at various life stages. Under the conditions of our experiment, 3600 m3 x hm(-2) was the optimal irrigation amount.