Abstract: The Relativistic Space-Charge Wave Theory has been developed systematically, and has been first in the world used to establish the Linear Theory of Free Electron Laser. It reveals the physical essentials of the electron bunching process and the particle-wave interaction mechanisms in FEL. The new physical concept " Relativistic Dibunching", which is a general phenomena in FEL and very important, has been presented and studied in detail, and some other new concepts and new suggestions have also been presented. This paper serves a overview of the works done recently by the author.