Due to the wide application of information and communication technology, the modern power grid has become a multi-dimensional heterogeneous complex system with real-time perception, dynamic control and information service, namely cyber physical power system(CPPS). Under the background of the deep integration of cyber system and power system, the power grid faces more network threats. In this context, a quantitative risk assessment method of grid cyber physical system under false data injection attack was proposed in this paper. Against the attack a power information security model for data flow delivery is established, which divides the attack into two processes: First, selecting the substation injection of false data, then based on the communication topology, a flow delivery path is constructed to simulate the attack source information transfer model. In addition, the attack graph is used to quantify the attack source information transfer model, and the potential consequences of the power system are quantitatively evaluated based on the optimal load reduction strategy under failure. Finally, through the case study, the weak nodes of the system were determined considering the factor of cyber-attack. It verifies the feasibility and effectiveness of the proposed method.