The utility model relates to an automatic open-close device for back door of hinge under fitted tipping wagon. The utility model consists of a back door hinge, a connecting rod, a lever arm, a hook and a bumper, wherein the back door hinge is arranged at joint between the lower end of back door and bottom end of a wagon box, and the back door of the wagon box can be tightly closed by adjusting the connecting rod to hit the lever arm under level situation of the wagon box; when a cylinder jacks the wagon box, the connecting rod pulls the back door to open through the lever arm and form an inclined unloading surface together with bottom of the wagon box to smoothen cargo unloading; when the wagon box is recovered to the original position, the connecting rod hits the lever arm to close the back door, and in the mean time, the bumper presses the hook to hook on to the back door and cause to door closed tightly. The utility model can effectively solve the problems of untight closing of back door of tipping wagon and unsmooth unloading of the prior art.