Abstract: As the fourth generation light source hard X-ray SASE free-electron laser has many unique properties superior to the 3rd synchrotron radiation: higher brightness (ten orders of magnitude of peak spectral brightness and three~five orders of magnitude of pulse duration shorter than the 3rd synchrotron radiation currently achieved), shorter pulse(two orders of magnitude of pulse duration shorter than the 3rd synchrotron radiation, down to sub-pi-cosecond) , shorter wavelength down to 0.1nm etc.. SASE issbased on the principle of high gain free electron lasers, and technologies of photoinjector and linear accelerator. The development history of SASE, the basic theory, the typical configuration, the main physical characteristics and the requirements for electron beam is described.