In order to meet the power grid operation safety requirements of "real-time analysis and pre-regulation", a power grid operation risk assessment scheme considering second and third defense lines of security and stability is proposed to solve the security problem from a lack of real-time interaction between automatic-safety device’s control and power gird operation. In this scheme, on duty control strategy and fixed value of automatic-safety device is described with XML language, and it is transmitted to the online dynamic safety assessment system in real time. The online dynamic safety assessment system establishes the control strategy model of the automatic-safety device based on the starting logic, action logic and control equipment, and then evaluates whether the on duty control strategy of the automatic-safety device can guarantee the safe and stable operation of the power grid under the predicted failure scenario based on the real-time operation data of the power grid. The online dynamic safety assessment system further calculates the level of power safety accidents caused by the action of the automatic-safety device according to the relevant provisions such as decree No. 599. Finally, the risk assessment of the automatic-safety device action consequence is realized. The scheme is applied in Guangdong Power Grid for the first time. The effect shows that the scheme can take into account the actual impact of second and third defense lines’ on duty control strategy and improve the accuracy of the online dynamic security assessment results. This work is supported by Science and Technology Project of China Southern Power Grid Co., Ltd. (ZDKJXM20180078).