Abstract: A GML based lightweight WebGIS system called PKGML is introduced in the paper. PKGML has basic GIS functions and doesn't need complex GIS server, and it possesses the feature of functional extensibility. Firstly, the architecture of PKGML is expatiated. PKGML consists of two parts: PKGML Kernel and PKGML Extension. PKGML Kernel has six kinds of core GIS functions: Service Accessing, Data Parsing, Caching and Indexing, Memory Data Pooling, Data Structure and Algorithms, Display and Interaction. PKGML Extension includes PKGML Applet for WebGIS applications, PKGML Portable for mobile GIS applications and PKGML Component for other complex GIS applications. The implementation details of PKGML based on Java are presented, and finally a sample of digital PKU campus system is used to illustrate the application of PKGML.