Extension is a new subject which solves sontradictory problems. It studies the extensive possibility of things and the rules and methods of exploitation and innovation with formalized model. The extension control theory and method has provided the basis theory and method for people to solve the existent contradictory questions in the automatic control system such as stability, accuracy and speed.This paper briefly introduces the related basic concepts of extension control, such as the structure of canonical extension control, extension set of character status of basic extension controller, and general extension control algorithm, and so on. Also presents the extension applition in power systems. The future work and prospect of development are described.