A double-station gantry combined machining system allowing automatic overturning of workpieces, comprising: a first mobile gantry machining unit, a second mobile gantry machining unit, an overturning platform, a first workpiece fixing table, a second workpiece fixing table, a guide rail and a workbench, wherein the overturning platform is arranged on the workbench and positioned between the first gantry machining unit and the second gantry machining unit; the first mobile gantry machining unit comprises a first cross beam, a first workpiece grabbing structure, a first double-drive screw transmission structure and a first side rocker unit, and the second mobile gantry machining unit comprises a second cross beam, a second workpiece grabbing structure, a second double-drive screw transmission structure and a second side rocker; the first workpiece grabbing structure is arranged on a side, opposite to the second cross beam, of the first cross beam, the first double-drive screw transmission structure is arranged on another side of the first cross beam, the second workpiece grabbing structure is arranged on a side, opposite to the first cross beam, of the second cross beam, the second double-drive screw transmission structure is arranged on another side of the second cross beam, the first double-drive screw transmission structure drives the first side rocker to move upwards and downwards, and the second double-drive screw transmission structure drives the second side rocker to move upwards and downwards; the first workpiece grabbing structure is used for grabbing a workpiece on the first workpiece fixing table or the overturning platform and placing the workpiece on the overturning platform or the first workpiece fixing table, the second workpiece grabbing structure is used for grabbing a workpiece on the second workpiece fixing table or the overturning platform and placing the workpiece on the overturning platform or the second workpiece fixing table, and the overturning platform is arranged on the guide rail and used for overturning the workpiece. a first mobile gantry machining unit, a second mobile gantry machining unit, an overturning platform, a first workpiece fixing table, a second workpiece fixing table, a guide rail and a workbench, wherein the overturning platform is arranged on the workbench and positioned between the first gantry machining unit and the second gantry machining unit; the first mobile gantry machining unit comprises a first cross beam, a first workpiece grabbing structure, a first double-drive screw transmission structure and a first side rocker unit, and the second mobile gantry machining unit comprises a second cross beam, a second workpiece grabbing structure, a second double-drive screw transmission structure and a second side rocker; the first workpiece grabbing structure is arranged on a side, opposite to the second cross beam, of the first cross beam, the first double-drive screw transmission structure is arranged on another side of the first cross beam, the second workpiece grabbing structure is arranged on a side, opposite to the first cross beam, of the second cross beam, the second double-drive screw transmission structure is arranged on another side of the second cross beam, the first double-drive screw transmission structure drives the first side rocker to move upwards and downwards, and the second double-drive screw transmission structure drives the second side rocker to move upwards and downwards; the first workpiece grabbing structure is used for grabbing a workpiece on the first workpiece fixing table or the overturning platform and placing the workpiece on the overturning platform or the first workpiece fixing table, the second workpiece grabbing structure is used for grabbing a workpiece on the second workpiece fixing table or the overturning platform and placing the workpiece on the overturning platform or the second workpiece fixing table, and the overturning platform is arranged on the guide rail and used for overturning the workpiece.