The invention relates to the technical field of a high voltage power transmission and transformation system, and discloses a testing device and method for a high voltage supply system loop. The testing device for a high voltage supply system loop comprises a three-phase test power supply, a star point mode device, phase sequence detectors and a toggle switch, wherein the star point mode device is three switching devices which are connected in the same phase sequence; a first end of the star point mode device is connected with the three-phase test power supply; a second end of the star point mode device is connected with an inlet wire breaker A of a tested distribution system; a third end of the star point mode device is connected with an inlet wire breaker B of the tested distribution system; and the inlet wire breaker A and the inlet wire breaker B are connected through copper contacts at two sides of a buscouple lifting cabinet breaker C in the disconnected state for the tested distribution system. The testing device and method for a high voltage supply system loop can provide the same phase sequence power supply for the tested distribution system through safe voltage, and can overcome the defect that an existing cable checking is easy to go wrong; the workload is reduced and the test is safe and reliable by means of comparison of the phase sequences of the second and third phase sequence detectors with the test phase sequence of the first phase sequence detector.