Abstract: Diverted water volume from the Yellow River to Ningxia irrigation area has continued to drop since 2000, having a significant impact on the development of irrigation in Ningxia. In order to determine the effect of decreasing diverted water volume on pollution of irrigation return flow in the region, the Yinbei Irrigation Area comprising of two typical drains (Yinxin and the Fifth drains) was used as a case study. Firstly, the yearly regularity of the amount of farmland irrigation return flow and the sources of pollution in the Yinxin and Fifth drains were determined. Furthermore, the evolution of ammonia nitrogen and total phosphorus concentrations during irrigation period in the drains were analyzed using the Mann-Kendall method. Meanwhile, farmland pollution load was calculated using a hydrology statistical analysis method. Finally, the effect of the decrease in water diversion on pollution of irrigation return flow in Yinbei Irrigation Area was analyzed and discussed. The results showed that the discharge processes in the drains were consistent with water diversion processes in the irrigation area, while irrigation return flow pollution mainly occurred in the stage of irrigation. Since 2000, the amount of irrigation return flow fell significantly in the Yinxin and Fifth drains. This change was largely in line with the diverted water volume from the Yellow River to the irrigation area. Furthermore, the water quality of irrigation return flow was improved during irrigation period in the Yinxin and Fifth drains. For the Yinxin drain, the concentration of total phosphorus apparently decreased and the concentration dynamics of ammonia nitrogen stabilized during irrigation period. Similarly, there were decreasing trends in the concentrations of ammonia nitrogen and total phosphorus in the Fifth drain during irrigation period. Compared with the period prior to 2004, ammonia nitrogen and total phosphorus load in the Yinxin drain dropped respectively by 35.3% and 41.7%, while that in the Fifth drain dropped by 71.2% and 63.7%. Correlation analysis revealed that non-point pollution load from farmlands was positively related with the volumes of irrigation return flow in the Yinxin and the Fifth drains. It was concluded that the control of the diversion of water volume and the adoption of agricultural water-saving techniques contributed to the control of pollution of farmland irrigation return flow in Yinbei Irrigation Area.