Abstract: As essential component of the PSⅡ reaction center, Cyt b559 is a heme protein composed of α- and β-subunit. In this paper, the molecule components and the characteristics of Cyt b559 are introduced briefly. Cyt b559 possesses the characteristics of redox and the capability of electron transferring, and it is presumed that Cyt b559 is involved in both mechanisms of protecting PSⅡ reaction center from damage and electron cycle around PSⅡ.