Abstract: This paper puts forward a “plug-in GIS development method” which divides the GIS software development into two parts:host development and plug-in development.Each part will be developed to comply with the COM specification.It will give the host a standard interface by using a plug-in connector,a plug-in can plug into the host by installing the plug-in.It realizes the“plug in and play”by the two steps. This paper introduces four aspects:the scheme of plug-in GIS development,the plug-in development,the standard interface creation of host system and the development of plug-in. In the end, a case is demonstrated to explain the whole development process.