Abstract: The voltage sag will bring big economic loss every year, so the monitoring of voltage sag in whole power grid is of great significance. In this paper, an improved monitoring allocation method by considering the propagation law of voltage sag is proposed. Firstly, power system partition principles and method for monitoring allocation are proposed based on the analysis of propagation law of voltage sag. Then, bus\|MRA matrix and line\|MRA matrix are defined, and the latter is calculated by interpolation and fitting method, which maintains the continuity of lines. On this basis, monitoring allocation model is built. In the end, taking IEEE 30\|bus system as an example, the allocation method is realized by MATLAB and simulated in PSCAD. The results verify the correctness and feasibility of proposed method, and the accuracy of recorded voltage sag information propagating through transformers can be ensured.