Geographic Information System (GIS) technologies and remote sensing (RS) technologies have developed rapidly in recent years, and they have been widely used indaily life of ordinary people. The combination of these two remarkable technologies isuseful for location decision making and has been applied in different kinds of study cases.Guangyuan is one of the fastest developing cities in the southwest of China. Especiallyafter the Wenchuan earthquake in 2008, the development in economic and urbanreconstruction increased rapidly. Many infrastructure constructions and the reformprojects are in progress. At the same time, China's urban sewage treatment facilities areseriously inadequate. Only a small percentage of sewage has been treated by sewagetreatment plants in China. So the purpose of this study is to select an optimal site for asewage treatment plant in Guangyuan in a scientific way.In this particular study, based on GIS software and GIS-based multi-criteria analysis(MCA), a decision making model has been built for optimal site selection for a sewagetreatment plant. Two types of data were used in this study. Digital elevation model andsatellite image, several factor maps and constraint maps were created for the final analysis.The analytic hierarchy process was used to apply the weights for each factor along withformula method, in order to get the best result and find the optimal site. Finally, a MCAmodel has been made to be an example for future similar studies.In the end, an optimal site has been selected. Although aims are achieved in this study,there are still some limitations in different perspectives of the study. In the future, moreprecise data can be used in MCA studies, data limitations could be reduced with thedevelopment of RS techniques. In the future, more similar studies will be finished, whichmeans more scientific papers can provide reliable references of determining the criteriaand weights.