The utility model discloses a jacket support structure for expanding an offshore platform. The structure comprises a jacket main leg where a jacket steel pile is inserted, wherein an artificial leg upper horizontal support pipe, an artificial leg lower horizontal support pipe and an artificial leg inclined support pipe are welded on the external side of the jacket main leg to formed a frame structure to support an artificial leg main pipe; the top part of the artificial leg main pipe is provided with an artificial leg top base plate; the external side of the artificial leg main pipe is fixed with a rubber fixed base plate through a longitudinal supporting rib plate and a transverse supporting rib plate; and the rubber fixed base plate is provided with berthing rubber through a fixing bolt. With the adoption of the structure of two main piles and two artificial legs, a form of a four-point supported chunking main upright is formed, therefore, the safety for chunking during offshore installation is improved; the deck area and the hoisting weight of newly increased chunks can be improved by 1.5 to 2.0 times; meanwhile, the requirements on construction resources can be reduced, thereby reducing the total investment of an overall project and improving the yield of adjusting and developing an oil well.