The invention discloses a combined vibration isolation device installed in a transformer oil tank and a design method. The device is installed at each of the joints of a transformer body in the transformer oil tank and a box body, and comprises a container, a metal rubber vibration isolation pad and a top cover, wherein the container is located at the lower portion, the metal rubber vibration isolation pad is installed in the container, and the top cover is arranged above the container; the periphery of an opening part of the upper end of each container is connected with a flange plate; corrugated pipes are connected between each flange plate and the corresponding top cover; and a damping spring vibration isolator is arranged between each metal rubber vibration isolation pad and the corresponding top cover. The combined vibration isolation device performs vibration isolation in allusion to different resonant frequencies of the transformer body in a manner of series connection combination of the metal rubber vibration isolation pads and the damping spring vibration isolators, and thereby, vibration transmission is weakened to the maximum degree, the vibration isolation bandwidth isincreased, and the goal of reducing the noise of the transformer is better achieved.