The invention discloses a horizontal bearing reinforcing body of a soft foundation cast-in-place pile and a construction method thereof. The device comprises one group of wing plates with the same structure and size and a hoop, wherein the wing plates are uniformly arranged at the periphery of the hoop, and small holes are arranged in the wing plates; the wing plates are in a rounded-trapezoid-like shape, and clamping grooves are arranged in the short edges of the wing plates; projecting teeth are uniformly arranged in the hoop in the peripheral direction, the number of the projecting teeth is consistent with the number of the wing plates, and the wing plates are hinged to the hoop through the clearance fit of the clamping grooves and the projecting teeth; and the wing plates can rotate relative to the hoop in a 90-degree range. The long-lasting problems that the single dependence of the horizontal bearing capability improvement of the cast-in-place pile on the diameter increase of the pile causes cost wastes and a specially-shaped pile foundation has to depend on special construction equipment are solved. According to the method, the horizontal bearing reinforcing body is jointly formed by adopting the wing plates, the hoop and grouting concrete, the improvement of the horizontal bearing capability of the cast-in-place pile is realized, the vertical bearing capability is also enhanced, and special pile foundation construction equipment is not required; and the mounting method is simple, efficient and rapid.