The invention relates to an automatic packaging device. The automatic packaging device comprises a machine frame, wherein a first sliding plate and a second sliding plate are slidably connected to themachine frame, the side parts, right opposite to each other, of the first sliding plate and the second sliding plate are correspondingly provided with heat sealing knives, a first air cylinder is arranged on the first sliding plate, and a second air cylinder is arranged on the second sliding plate; a rotating disc is arranged below the first sliding plate and the second sliding plate, and meshingteeth are arranged on the outer wall of the rotating disc; a first gear meshes with the meshing teeth, a vertical shaft is connected to the first gear, and a one-way bearing is connected between thevertical shaft and the first gear; the first sliding plate is connected with a first mechanism used for enabling the vertical shaft to rotate, a supporting ring with adjustable diameter is arranged above the first sliding plate and the second sliding plate, and the supporting ring is connected with an adjusting mechanism used for adjusting the diameter of the supporting ring; and a hopper is arranged above the supporting ring, a discharging plate is connected to the bottom of the hopper in a sliding mode, and a second mechanism is connected between the discharging plate and the first sliding plate. According to the scheme, food can be conveniently put into plastic bags with different sizes, and packaging is not limited by the size of the plastic bags.