A mechanical reliability testing platform for tri-post insulators in a GIL device includes a horizontal-GIL-arrangement-form fixed-tri-post-insulator mechanical reliability verification testing platform for a horizontal dynamic insertion and extraction test, and a turning-GIL-arrangement-form fixed-tri-post-insulator mechanical reliability verification testing platform for a vertical dynamic insertion and extraction test. A driving unit is employed to realize the insertion and extraction of the conducting rod of the sliding-tri-post-insulator GIL form unit at the contact holder, so as to simulate the reciprocating forces on the fixed tri-post insulator induced by the thermal expansion and contraction of the pipe during the actual operation of the GIL, and simulate the working condition of the fixed tri-post insulator under abnormal forces when the GIL experience foundation settlement.