The invention discloses a foundation pit supporting structure based on an old pile row and a construction method thereof. The foundation pit supporting structure comprises an enclosure structure, a crown beam, a middle beam and an internal supporting structure used for balancing the water-soil pressure from the enclosure structure, wherein the crown beam is arranged at the top of the enclosure structure, the middle beam is arranged below the top of the enclosure structure, and the internal supporting structure is arranged on the inner side of the enclosure structure and is connected with the enclosure structure through the crown beam and the middle beam. The construction method of the foundation pit supporting structure comprises the steps that a soil body is reinforced; an original crownbeam is removed, and an underground diaphragm wall and vertical supports are constructed; the jet-grouting pile crown beam and a horizontal support are constructed; the soil body is excavated, and a net is hung on the inner side of the old pile row for concrete pouring; the middle beam and the horizontal support are constructed; and the step of excavating the soil body, the step of constructing the middle beam and the horizontal support and other steps are repeated till a foundation pit is excavated. According to the foundation pit supporting structure based on the old pile row and the construction method thereof, the residual value of an old enclosure structure can be used, and the influences of construction of the enclosure structure on existing buildings are eliminated; and the foundation pit supporting structure based on the old pile row and the construction method thereof have the advantages that the old pile row is used for saving energy, the construction period is shortened, theconstruction cost is reduced, and the influences of construction on the environment are reduced.