The utility model discloses a down-draft preventing device for a gas heater, which comprises a pipe body, wherein a rotation shaft passes through the pipe body, the rotation shaft is hinged with wind plates, a technical key point of the down-draft preventing device is that: one side of an air inlet port of the pipe body is provided with an inner liner plate, the inner liner plate is provided with vent holes, and the wind plates cover the vent holes to form uni-directional closing; the pipe body comprises an upper pipe body and a lower pipe body, and the inner liner plate is arranged between the upper and the lower pipe bodies; and the inner side of the pipe body which is close to an air outlet is provided with a positioning pin for restricting the rotation angle of the wind plates. The wind plates of the down-draft preventing device can completely, efficiently and tightly cover the vent holes of the inner liner plate. The down-draft preventing device has the advantages of simple structure, low cost, good tightness, small opening resistance, etc.