The invention discloses a method for accurately measuring wide-area power grid PMU (Phasor Measurement Unit) data time delay, and relates to the field of real-time dynamic monitoring of electric power systems. At present, the wide-area power grid PMU data time delay cannot be accurately measured. The method includes the following steps of: arranging a simulated front-end system at a WAMS master station terminal; transmitting-receiving mirror images by a port on the PMU data communication side of an interchanger between the PMU and the simulated front-end system or between the PMU and the WAMS interchanger to an idle port; access the mirror image port into a collection internet access of a network recorder which records moment of data passing through the interchanger; analyzing to obtain the time information of a required data interaction process according to the original communication data collected by the network recorder; calculating the wide-area power grid PMU data time delay according to the time information of the data interaction process. The technical scheme does not influence the existing data interaction of WAMS and PMU, and can obtain the accurate PMU data time delay.