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The objective was to establish the root length density (RLD) distribution relation model of maize at different nitrogen rates, and explore the effects on RLD of maize under drip-irrigation with plastic film in Ningxia. The relationship was improved between the spatial distribution characteristics of maize root system and nitrogen absorption. The interaction was promoted between maize root and soil. The nitrogen fertilizer application rate of maize was optimized under the condition of water and fertilizer integration. There were five nitrogen fertilizer application rates and the treatment of no application of N fertilizer and no mulching as control (CK) in this experiment. The experiment plot was in farm of Pingjibao which was located in Yinchuan City, Ningxia, Northwest China. The RLD of drip irrigated maize was measured. The model of relative root length density (NRLD) was established and verified at different N application rates. The results showed that the RLD distribution of drip-irrigation maize was decreased with the increase of depth of soil layer in the N fertilization treatment. The vertical distribution of root length in the 0~20cm soil layer was the largest. The RLD distribution range of maize was expanded significantly with the increase of N fertilizer application rate. The NRLD value of maize in all treatments were consistent with cubic polynomial function. The determination coefficient of simulation curve was 0.951. The results of the evaluation showed that determination coefficient of linear relationship between simulated and observed values of NRLD was 0.845, and the root mean square error (RMSE) was 0.248. The fitting result of cubic polynomial function had a good effect. But it should be noted that the polynomial model could not guarantee that the value of NRLD reached zero at the Zr was equal to 1. Therefore, the cubic polynomial model of NRLD maize was optimized and verified. The verification results of the optimized model showed that RMSE in all nitrogen treatments was not more than 0.308, the standardized root mean square error (nRMSE) were 0.242, 0.193, 0.184, 0.226, 0.208 and 0.273, and determination coefficient values were 0.903, 0.953, 0.920, 0.944, 0.962 and 0.898, respectively. It not only had a higher fitting degree, but also solved the problem that the value of NRLD reached zero at the Zr was equal to 1. It was more suitable to simulate NRLD distribution of drip-irrigation under plastic film of maize for different nitrogen rates. All these results given can also provide a theoretical reference for nutrient absorption and fertilization management of drip-irrigated maize root system in the irrigation area of Ningxia.