The interconnected power grid has developed rapidly and gradually formed a whole. How to accurately evaluate the reliability of a certain area of the grid under the interconnection of information in the case of information isolation is a difficult problem for the safe operation of the interconnected power grid. In response to this problem, this paper proposes a reliability assessment method for interconnected grid regional grid operation based on PMU monitoring. The method uses the boundary PMU measurement data to estimate the external network static equivalence model to simplify the interconnected power grid, and adopts the interconnected grid asynchronous iterative method to update the external network static model equivalent parameters to ensure the accuracy of the reliability calculation results. The results of the example based on the IEEE-RTS-96 node system show that the proposed method can quickly and accurately calculate the reliability index of the interconnected power subsystem without relying on the actual topology information of the interconnected power grids. This work is supported by National Key Research and Development Program of China (No. 2017YFB0902800) “Research on Key Techniques of Intelligent Distribution Network Operation based on Micro Synchronous Phasor Measurement”.