Abstract: Based on the analysis of the research status of water quality of domestic and foreign scholars, the characteristics of wastewater discharged by small and medium-sized enterprises were clarified, the historical water quality data of wastewater discharged were classified and correlated, and the relationship between the easily monitored water quality index and water quality grade was analyzed. The water quality monitoring model of wastewater discharged by small and medium-sized enterprises based on water quality grade classification was proposed. The decision tree algorithm was used to build the water quality classification model. It can effectively replace the traditional equipment with the high purchase price and high operation cost, and provide a convenient and scientific wastewater discharge monitoring system for enterprises, governments and other relevant departments. Finally, the model was verified with the river water quality data of a city, and the accuracy rate was 78.56%,which proved the feasibility of this method,and could provide all-weather on-line monitoring for small and medium-sized enterprises to discharge wastewater, and provided a scalable monitoring system for wastewater discharge of small and medium-sized enterprises with wide distribution, a large proportion and difficult supervision.

Key words: water quality classification, decision tree algorithm, small and medium enterprises, water quality monitoring of wastewater discharge