In recent years, water pollution incidents occurred frequently in China, which has caused widespread concern in society. A number of water pollution diffusion simulation systems are developed in order to grasp the actual situation of water pollution, and then execute a more scientific management for the water environment. However, this kind of systems developed with the component technology is so complex that most of them have poor software scalability and is difficult to maintain after delivery. In order to ensure system scalability, it needs a kind of loose coupling mechanism to combine water pollution diffusion model, GIS and visualization modules in one system. Plug-in technology can reduce the coupling between system modules. Software developed with plug-in technology can extend its functionalities without modifying its main program. In this paper, the general structure of plug-in software is described in the first place, and then the designs of the host program and plug-ins are showed, which are used to drive the development of the plug-in Taihu Lake water pollution diffusion simulation system. This is a valuable attempt to develop customizable GIS with plug-in technology. At last, the plug-in Taihu Lake water pollution diffusion simulation system is verified to achieve three-dimensional visualization of pollutant concentration and flow of Taihu Lake by examples. The plug-in Taihu Lake water pollution diffusion simulation system developed with plug-in technology (a kind of loose coupling mechanism) that integrates water pollution diffusion model, GIS and visualization modules not only simulates intuitively spatial distribution of water pollution and dynamic evolution process of water pollution diffusion in order to provide effectively decision support for water pollution control in Taihu Lake, but also has excellent dynamic scalability that extends system functionality by adding new plug-in without affecting the system that has been deployed.