The invention discloses a wide-area relay protection method based on measurement conversion state estimation. The wide-area relay protection method based on measurement conversion state estimation can make full use of high-accuracy, low-sampling-period and wide-area measurement data of a phasor measurement device, judge the accurate position and fault type of a faulted branch and determine the state of a system. Firstly, the measurement data of each branch are collected, the suspected faulted branches are determined, measurement bad data are eliminated, the wide-area protection algorithm is further executed, equivalent measurement of non-faulted branches and faulted branches is obtained respectively through the measurement changing method, a measurement equation of equivalent measurement is built, the wide-area relay protection method based on measurement conversion state estimation is built to obtain the state quantity of the system, and the faulted branch and the position of the faulted branch are determined.