The invention provides an automatic open-close flap gate provided with a longitudinal oblique shaft. The automatic open-close flap gate comprises a barrage, a gate pier, a gate, a downstream panel, anupstream panel, a baffle bar, the oblique shaft, an oblique shaft support plate, a drainage hole, a bracket, a bracket hole, an upper oblique shaft hole, a lower oblique shaft hole, a floating box, floating box guide rails, a water inlet hole, floating box turning wheels, upper hook plates, lower hook plates, a sliding door, vertical plates, a horizontal baffle, a control board, a control board groove, sliding door turning wheels, an adjusting screw, a gate cavity, a contact support, sliding door guide rails and a limiting block. The flap gate is automatically opened and closed in the lateraldirection, and mounting of oil pressure opening and closing facilities is not required; the gate constitutes a damping system, additional damping is not required, the problems that the gate is damaged and the service life of the gate is affected due to the fact that the flap gate is flapped, vibrated and impacted in the turnover process can be solved, the problem of shielding and blocking due tosuspending of the gate in the middle of the gate chamber after the horizontal flap gate is opened is solved, water discharging is smoother, and the potential risk that gate closure is affected by thefact that floating objects are hooked on the gate is eliminated.