With deterministic-rule based power grid operation control， it is difficult to coordinate the stability，security and economic of power grid. The development and engineering application of online security and stability defense technology is the base for online operational security risk assessment of power systems. The status quo related to online operational security risk assessment is reviewed. Base on the definition of engineering risk， the meaning of operational security risk and the characters and requirement to risk index are discussed. Besides， the distinguishing features of online operational security risk assessment of power systems is analyzed.