The invention discloses a digital protecting tester which comprises a PWF main control board, a FT3 board, an input measuring board, an output measuring board, an Ethernet interface and an optical fiber interface. An automatic closed loop test system comprises a computer, a network connecting device and the digital protecting tester, and the digital protecting tester is connected with a protecting device by the optical fiber interface and is used for sending a sampling value and issuing/subscribing GOOSE information. An automatic closed loop test method comprises the following steps of: establishing a test task according to a test plan by a tester, automatically configuring GOOSE parameters and automatically reading/writing the set value of a relay protecting device; simulating a fault according to the test task and the protecting set value and automatically analyzing GOOSE data to obtain a test result; automatically reading the sampling value of the relay protecting device; automatically reading the switching-off/switching-on report of the protecting device; and automatically generating a complete test report by software after completing test. By the invention, the automatic test can be completed, the test efficiency is improved, and the workload of the tester is lightened.