In the wide area relay protection of power system, reasonable and correct protection area division is the key to improve the selectivity and sensitivity of relay protection. In modern power grid, based on the principle of longitudinal comparison, the partition method of wide-area protection area is carried out, the protection area of each IED is determined according to the relationship between IED and line or bus, and the division principle is relatively simple. With the gradual formation of interconnected large-scale power grid, the network structure is complicated and the topology structure is changeable, simple protection area division based on the principle of protection area division can no longer meet the requirements of wide-area relay protection area division of power system. Therefore, this paper proposes a new method of t-test area partition based on rough set theory. When the system structure changes, this method can use the function of reducing redundant information by rough set theory to find the protection range dynamically; use t-test method to determine the wide-area protection range of each IED; verify it by CEPRI-36 node simulation system model. This method does not depend entirely on the structural relationship of the system to divide the protected area, and has better applicability to the network with changeable structure. This work is supported by Science and Technology Development Program of Jilin Province — Research on On-line Monitoring and Analytical Technology of Transmission and Transformer Equipment Operation Status (No. 20160307014GX).