Abstract: Crop root length and soil moisture distribution are important determinants of crop root water uptake potential. Crop root length changes with growth stage, which also requires changes in soil moisture environment. Therefore, establishing a root growth model to predict crop root growth conditions under normal water use has theoretical and application significance regarding the determination of irrigation quota and soil moisture environment indicators, which are required to design appropriate local irrigation technology. However, current root growth simulating models are more suitable for the determination of irrigation quotas of whole irrigation technology. These models, which are mostly statistical, can not completely reflect the relational mechanisms of the growth of plant root system, crop water use and soil moisture environment. In view of the above and, a root-canopy water balance combined with crop coefficient vs. leaf area relationship model, root length density distribution function and root water uptake efficiency function, a cotton root growth model was developed based on dynamic programming theory and verified by experimental results of a barrel-cultivated cotton. The main results of the study showed that the model well accounted for the effects of root growth factors such as soil moisture environment, atmospheric transpiration rate and leaf area, which revealed to a certain extent the mechanism of crop water use due to root growth. The growth characteristics of cotton root length simulated by the model were consistent with measured dates in the barrel experiment. When multi-year average monthly mean reference evapotranspiration (ET0) was used as input condition, the overall error of the simulation result was 15.41%. Therefore the model was applicable in engineering designs. Based on sensitivity analysis, the established model well reflected the synchronization between the growth of cotton root and leaf, as well as the water balance between root and leaf after entering the reproductive period. The sensitivity of cotton root growth to changes in soil moisture environment was higher than to changes in leaf area, reflecting the processes of cotton root growth and the feasibility of the modeling method. The research significantly improved the design theory of irrigation systems for the development of localized irrigation technology.