Roots are very important part of plant substance metabolism and information exchange system. The development situation and vitality of root system effects greatly crop shoot growth and yield. While soil moisture decrease, roots will detect firstly soil moisture changes and respond actively to water regulation, which is beneficial to drought resistance and good yield. Therefore, root system research has become the highlight in crops drought-tolerance and water saving researches in recent years. In order to understand the dynamics of maize root water uptake under controlled root-divided alternative irrigation (CRDAI), a field experiments was carried out in 2010 and 2011 seasons to investigate root distribution and dynamics of maize root water uptake at different sites (furrow bottom, slope and top ridge) under CRDAI. Results indicated that spatial distribution of root length density (RLD) was influenced obviously by alternative wetting and drying in maize root zone. The maximum RLD at vertical direction occurred at soil layer of 10-20 cm, and then RLD decreased gradually as depth increased. The roots at top ridge site had maximum penetration depth. Root senescence started at early grain filling stage, and senescence rate of roots at furrow bottom was greater than that at top ridge. Maize roots distributed asymmetrically on both ridge sides under CRDAI. RLD at horizontal direction declined generally in order: top ridge, slope, and furrow bottom. The RLD distribution models at different soil layers and growth stages were quite different. There were no regularities to fit RLD distribution at soil layer of 20-50 cm, because of the obvious influences of frequently soil moisture changes. Maize root water uptake under CRDAI was mainly determined by soil moisture and RLD distribution. At same soil depth, the main contribution to total root water uptake may attributed to roots at top ridge, and minimum contribution come from roots at furrow bottom. The maximum contribution to root water uptake come from soil layer of 10-30 cm during vegetative growth stage. The root water uptake in soil layer of 20-70 cm contributed a main part of the total water absorption during reproductive growth stage, because of higher RLD value and water uptake rate in the soil layers. The percentage of root water uptake in soil layer of 20-70 cm to total root water uptake increased from 33.34% in vegetative growth stage to 55.48% in reproductive growth stage. Soil moisture content in the main water absorption layer was very crucial to meet crop water requirement. After grain filling started, the roots in top soil layer senesced gradually, while RLD in deep soil layer increased slightly. Therefore, water update from roots in soil layer below 70 cm depth was very important to good grain filling. The distribution of RLD and soil moisture in soil profile was very important to crop root water uptake and crop viability under water stress. The dynamics of the interaction among root water uptake, soil moisture and RLD distribution under CRDAI were investigated and analyzed in this paper, which is helpful for understanding crop water regulation mechanism under controlled alterative irrigation.