The invention relates to a double row pile and diagonal brace combined soft soil deep foundation pit supporting structure. An inner row pile and an outer row pile are arranged outside a foundation pit, a connecting plate, a top curing region, a pile block coupling beam and a vertical waterproof pile are arranged between the inner row pile and the outer row pile, and a vertical water-stop plate is inserted into the vertical waterproof pile; a pit bottom slurry soil fixing layer is arranged in the bottom of the foundation pit along the inner side of the inner row pile; a diagonal brace is arranged between the inner row pile and a basic raft board, and a vertical steel latticework column is arranged across the middle part of the diagonal brace; a bottom supporting pier is arranged at the connection of the diagonal brace and the basic raft board, and a waist beam is arranged at the connection of the diagonal brace and the inner row pile; and an anti-floating anchor and a main body structural pile are arranged under the basic raft board. The structure provided by the invention not only can enhance the stability of the foundation pit support structure and reduce the pit bottom rebound deformation, but also can prevent seepage failure of the foundation pit. The invention further discloses a construction method of the double row pile and diagonal brace combined soft soil deep foundation pit supporting structure.