Abstract: Surface water quality section of the tributaries of large rivers accounted for most of the sections; At the same time, it is an effective way to improve water quality to divide the task of reaching the standard into municipal and county administrative regions, and the evaluation, assessment and ranking of water quality in administrative regions at the same level have been carried out. However, the method of reaching the standard rate of the section, which is widely used in the evaluation of surface water quality in the county area, cannot meet the needs of ranking and assessment, and distinguish the quality of water between good and bad. Based on the statistical analysis of surface water measured in 7 provinces and cities in western China for 25 consecutive months, a set of monthly test results model of surface water quality in counties was established by using the natural logarithm method, the model could be used to rank the quality of surface water in municipal or provincial districts and counties. The application showed that the model was reasonable, reliable, and easy to understand. It is expected to promote the establishment of the scientific method of surface water quality evaluation, assessment, and ranking.