The disclosure provides a method and device for reliability assessment of a wide area protection system, solving the technical problem that a reliability research result of a current wide area protection system is only limited to reliability research of a communication network thereof, but a reliability analysis model for an overall system does not exist. The method provided by the disclosure includes: S1, acquiring an information acceptance rate d of a wide area monitoring center of a wide area protection system; S2, obtaining the accuracy psf(d) of wide area protection initiation, the accuracy pfz(d) of fault area positioning and the accuracy pfc(d) of fault element recognition according to the information acceptance rate d; and S3, multiplying the accuracy psf(d) of wide area protection initiation, the accuracy pfz(d) of fault area positioning and the accuracy pfc(d) of fault element recognition to obtain the decision accuracy Pdmp of the wide area protection system.