The invention provides a movable formwork for casting of a wide-body box girder and a construction method, and relates to the technical field of bridge construction. The movable formwork comprises a plurality of formwork units which are arranged in the axial direction of a bridge, wherein each formwork unit comprises two side area independent units and a middle area independent unit; the two side area independent units of each formwork unit are arranged in the transverse direction of the bridge; an upright column of each independent unit can stretch out and draw back along the vertical direction; portions, which are close to the corresponding middle area independent unit, of longitudinal and transverse beam systems and form board systems of the side area independent units of each formwork unit are turnover structures; and ground beams of the side area independent units are arranged in the axial direction of the bridge, so that a driveway is reserved below the side area independent units. By the movable formwork, materials and labor can be saved, the construction efficiency is improved, influences on road traffic are low, and construction safety is high.