The invention discloses an old wall supporting replacing construction method and a beam plate transformation bearing structure thereof. The method comprises the steps that firstly, bolt holes are formed in an old wall needing to be demounted and formed along the two sides of a structural beam below a structural plate in advance; then, supporting devices are installed along the two sides of the structural beam, and batten plates are arranged on the supporting devices; then, the part, below the supporting devices, of the old wall is demounted while the parts, connected with the two ends of the supporting devices, of the old wall do not need to be demounted. In this way, the part, below the beam plate bearing system, of the old wall can be demounted at a time, a stable frame structure formed through part of the old wall and the supporting devices can ensure that the wall is free of perpendicularity abnormalities and cracks, and thus the purpose of replacing the part below the beam, of the wall is achieved. According to the old wall supporting replacing construction method and the beam plate transformation bearing structure thereof, needed devices are simple in structure, and field construction tower dismounting is convenient; besides, the construction cycle is short, and installation is convenient; thus, the height of the section of a supporting and replacing beam can be reduced in construction, so that the space used for transformation supporting and replacing is greatly enlarged, cost is reduced, and wall cracks are reduced.