The invention provides a pneumatic automatic return device. The problem that in the prior art, the return speeds of doors with different weights are different is solved. The pneumatic automatic return device comprises a shell, an upper convex shaft, a lower convex shaft, a piston, a sealed air chamber and an adjusting device, wherein the shell is of a hollow structure, the upper convex shaft is arranged in a gear sleeve, one end of the upper convex shaft extends outwards from the end part of the gear sleeve, the upper convex shaft only rotates in the gear sleeve, the lower convex shaft is also arranged in the gear sleeve and does reciprocating rectilinear motion only in the length direction of the gear sleeve, the upper convex shaft is in contact with the lower convex shaft, acting force always exists between the upper convex shaft and the lower convex shaft, the rotating motion of the upper convex shaft is synchronous with the rectilinear motion of the lower convex shaft, one end of the piston is used for continuously propping the lower convex shaft against the upper convex shaft, the sealed air chamber is a sealed cavity formed by an upper sealing piece and a lower sealing piece in the shell, high-pressure air is arranged in the sealed cavity, the other end of the piston extends into the sealed cavity, and the adjusting device is used for adjusting the volume of the sealing cavity.