First, the water of Huiji River in Kaifeng City was chosen as sample water body of the sewage irrigation, and water quality monitoring and analysis were conducted using standard methods. Considering the shortcomings of TOPSIS method to be used in carrying out a comprehensive assessment, we put forward to make use of the vertical distance instead of Euclidean distance . Through the establishment of improved TOPSIS model, the safety assessment of sewage irrigation was conducted. It is indicated that, the water quality of Huiji River in Kaifeng City was lower than GB5084-2005 standard and it is not safe to irrigate or do other things due to containing a variety of heavy metal pollutants. The present findings provide a scientific basis for water management and pollution treatment and it plays a great role in the sustainable use of water resouces.