Abstract: This paper elucidates our understanding of the key characteristic quantity of high energy laser(HEL) system which is rapidly advancing in the world. Factors influencing the key characteristic quantity are analysed: the evolution history of principal laser and its reasons; as beam quality is the lifeline of HEL system, and to gain a better beam quality is a multilink-chain scientific engineering, technical challenges in the upper, middle and lower reaches along the technical chain, as well as the stability of the whole HEL system are discussed. In the end, by combining all the factors of contribution, the key characteristic quantity is expressed as the laser beam brightness on the target.