A method for diagnosing an electric brake system is disclosed. The method for diagnosing an electric brake system, which includes a reservoir, a master cylinder, a first backup flow path, a second backup flow path, a first cut valve, a second cut, a simulation device, a hydraulic pressure supply device, a first hydraulic flow path, a second hydraulic flow path, a hydraulic control unit, and an electronic control unit configured to control the hydraulic pressure supply device and the valves on the basis of hydraulic pressure information and displacement information of the brake pedal, comprising a first diagnosis mode of: closing the first cut valve and the second cut valve when the pedal effort is not provided to the brake pedal; operating the hydraulic pressure supply device to generate hydraulic pressure at the hydraulic control unit; measuring the hydraulic pressure in the first or second hydraulic circuit; and comparing the measured hydraulic pressure with a first reference pressure to diagnose abnormality of the valves.