The working principle of Flywheel Energy Storage Systems (FESS) is described. Then the FESS's key technologies are analyzed: FESS is an integrated system which has the feature of multidiscipline intersection. To improve its performance indexes, the rotational drag, electromagnetic coupling and created heating of FESS should be decreased. Therefore, the key technology affecting FESS includes flywheel design, magnetic bearings technology, high-speed rotor dynamics, motor/generator and energy conversion technologies, and long time maintaining vacuum technology. Lastly the FESS's application status is discussed: With the development of key technologies, the application of FESS has extended to the fields of UPS, grid, marine etc.