Grid is a new paradigm of Internet computing and is developing rapidly. Web service technologies have improved the interoperability of grid protocols and services, as well as providing a unified functional extension mechanism. But current research has not definitely solved the problem of system construction based on Web services in grid environment, especially on the method of dynamically assembling existing resource to realize user-oriented services. This paper introduces an Active Services model and its supporting platform in grid computing environment, which focuses on relieving the disorder condition in service creating, publishing and deploying and providing a new method to provide user-oriented Active Services in grid computing environment. Active Services are proposed to actively collect, organize, classify and manage the published services to create services resource view for users on application level, and dynamically assemble and execute those services to meet the user demands. The paper also presents the implementation of Compositive suppOrting and Management Platform (COMP), with an applied case of customized multimedia player generated on it.