The invention discloses a bottom die supporting system of a long-piled wharf cast-in-place beam and a construction method of the bottom die supporting system. The bottom die supporting system comprises a plurality of steel protection cylinders arranged on the surface of a pouring pile in the axis direction of a single-row pile foundation. A plurality of supporting brackets parallel to the axis direction of the single-row pile foundation are fixed to the surfaces of the steel protection cylinders in the circumferential direction. Shelving beams perpendicular to the axis direction of the single-row pile foundation are shelved on the supporting brackets located on the front sides and the rear sides of the steel protection cylinders. Side longitudinal beams which are shelved on the shelving beams and are parallel to the axis direction of the single-row pile foundation are arranged on the left sides and the right sides of the steel protection cylinders. The span length of the longitudinal beams is reduced by the diameter of one steel protection cylinder through the longitudinally-arranged brackets, and the mid-span moment, borne by the longitudinal beams, transmitted by the cast-in-place beam is greatly reduced; and the number of the side longitudinal beams and the number of middle longitudinal beams are each three. Compared with a traditional method, one longitudinal beam is additionally arranged to participate in stress; inclined struts are arranged at the two ends of each middle longitudinal beam and can effectively bear the construction load of a cantilever section of the long-piled wharf cast-in-place beam; and thus, the safety of the bottom die supporting system is more easily guaranteed.