Most online evolution of application depends on its rumtime environment. This paper addresses how to support online evelution by apllication server, which is considered as thired as third kind of system software, besides OS and DBMD. From view of requirements, evolutions of software can bediveded into four categories: evolutions that do not alter requirements, evolutions that alter funcional requirements, evolution that alter local constraint requirements, and evolutions that alter global constraint requirements. All changes at the requirement level should be mapped to changes at the implementation level. In our approach, implementation level entities, such as components and interceptore are responsible for online evolution. Evolutions in implementation level include adding, removing, updation, and reconfiguring the entities. One of the keys to our approach is to carefully distinguish states of components and interceptors, that is, whether they are in a ready, active, executing or evolving state. A well--designed architecture and feasible mechanisms for runtime instance loading are also keys to the solution. Based on this approach, an application server prototyper, named PKUSAS, has been implemented and is introducedin our paper.