The invention discloses a cast-in-place concrete beam and slab formwork supporting system and a construction method thereof. The cast-in-place concrete beam and slab formwork supporting system is characterized in that steel bar truss floor bearing plates are arranged, upper chord steel bars, lower chord steel bars and web bars form trusses, steel bar truss bottom formworks are detachably fixed to the bottoms of the trusses through connectors, beam lower adjustable supporting structures are arranged to perform adjustment according to the height, width, clear height of beams and the clear height difference of beams and slabs, and beam and slab formwork supporting is achieved. In the cast-in-place concrete beam and slab formwork supporting system, the steel bar truss bottom formworks are of detachable structures, the construction efficiency can be greatly improved, the beam lower adjustable supporting structures make the width of the beams and the elevations of beam and slab bases adjustable, the length requirement is met without a cutting-off method, a construction period can be greatly shortened, and construction costs can be saved.