The invention relates to a composite foundation composed of a caisson with a steel cavity and piles constructed by the reverse operation method, and a construction method thereof, belonging to the bridge construction technology. The composite foundation comprises a caisson, a foundation bed, and piles. The caisson is composed of a steel cavity, an outer wall, an inner wall with a partition wall structure, a caisson cover, and a caisson bottom platform, wherein the caisson cover is preserved with water pumping and gas delivery holes, which are doubled as inspection holes. The caisson is arranged above the foundation bed and placed inside water, bridge piers are arranged on the caisson cover, the periphery of the caisson bottom platform is connected with the piles constructed by the reverse operation method, and the piles are inserted into the foundation bed and the sea bed or river bed through sinking pile preserved holes. After a foundation trench is excavated, a ship is located by riprapping to form the foundation bed, and the foundation bed is leveled by a platform type foundation bed riprapping leveling ship. The piles are connected with the caisson by pouring concrete in the preserved holes in the caisson. The inventive composite foundation utilizes the buoyancy force of the caisson steel cavity and considers the combined action of the piles, soil and the caisson to share partial external load by the sea bed or the river bed. The composite foundation optimizes the pile foundation design, reduces settlement during bridge service period, and reduces construction cost.