Abstract: Aiming at the security risks of power grid frequency oscillations caused by the external transmission line faults from the eastern developed area, the architecture of automatic generation control between power grid and power plant is analyzed, and a method of online monitoring of the characteristics of primary frequency regulation(PFR) in thermal power units is proposed. Based on the hybrid energy management system, the schematic diagram of PFR online monitoring is designed. According to the engineering practice, the effect of PFR is measured in terms of different evaluation indexes. Considering the performance of ultra-supercritical units and the technology of time division, the control strategy of PFR online monitoring is established. The test results demonstrate satisfactory performance of the proposed online monitoring system. In addition, improvement is made by replacing the original post-faults analysis with pre-faults supervision, and the parameters of PFR can meet all the requirements of dispatching procedures.