In recent years, with the accelerated process of urbanization, the government has increased a large investments in urban sewage plants , and the overall efficiency of a sewage plant depends on whether the size of the sewage plant adjust to the development of the local economy, population and environment. This article is trying to make use of the theory of regression analysis to probe the small/mid scale sewage plants in Southwest of China between 2008 and 2011. Finally we’ve found that scale economy effect do exist in the small/mid scale sewage plants of Southwest area. It shows that the scale of the sewage plants in Southwest of China is appropriate for the local economy. Also, it's significant to design the scale of a sewage plant in other places.