The invention discloses a flattening machine for annular craters of a fire extinguisher cylinder. A spindle case is arranged at one end of a lathe body. An expansion rod extends out from the end face, facing the other end of the lathe body, of the spindle case. An expansion block is arranged on the side wall of the end of the expansion rod. The expansion rod is fixedly connected with a spindle in the spindle case. The spindle is driven by a motor and rotationally connected with the spindle case. A flattening device is arranged in the position, opposite to the other end of the spindle case, of the lathe body, and the flattening device is fixed to the end of a piston rod A of a piston A. A circular notch having the same axis as the expansion rod is arranged at the end, facing the expansion rod, of the flattening device. A circle of pressure rollers are evenly distributed on the inner wall of the circular notch and surround the axis of the circular notch. According to the structure, through the flattening machine, no crater can be seen after the fire extinguisher cylinder is coated with paint, and on the premise that the pressure bearing capacity of the fire extinguisher cylinder meets the production requirement, the comfort and attractiveness of the cylinder are improved, and the additional value of the flattening machine is increased.