The invention discloses a device and a method for testing the performance of a low-voltage side of a GIL three-post insulator, and belongs to the technical field of electrical equipment. The device disclosed by the invention comprises a high-voltage side conductor, a liner tube, a graphite ring, a low-voltage side metal extension piece and a grounding metal shell, wherein one end of the high-voltage side conductor is connected with an external high-voltage source through a plug connector; the liner tube is sleeved outside the high-voltage side conductor, and the GIL three-post insulator to betested is sleeved outside the liner tube; the graphite ring is arranged between the liner tube and the high-voltage side conductor; one end of the low-voltage side metal extension piece is connected with a low-voltage end metal insert of the GIL three-post insulator to be tested, and the other end of the low-voltage side metal extension piece is fixedly connected to the grounding metal shell through a bolt; and the grounding metal shell is a hollow cylinder and is provided with a through hole on the wall. The device and the method for testing the performance of the low-voltage side of the GILthree-post insulator is simple in structure and convenient to use, and can improve the electric field of the low-voltage side of a three-post insulator supporting leg, so that the examination requirement of the electric performance test on the low-voltage side of the three-post insulator supporting leg is enhanced, and the product quality level is improved.