The utility model discloses a pneumatic locking and expansion fixed structure for auxiliary supporting to a bearing by upper and lower horizontal shafts in a six-axis and four-side moulder. The pneumatic locking and expansion fixed structure comprises a dovetailed pressing block locking pushing rod (1), a S140 cylinder joint (3), a cylinder (4), a cylinder seat (7), a locking gear I (8), a pin shaft (9) with a hole and a locking gear II (10) and the like, wherein the dovetailed pressing block locking pushing rod (1) is connected with the cylinder (4) through the pin shaft (9) with the hole and the S140 cylinder joint (3); the cylinder (4) is arranged on the cylinder seat (7); and the locking gear I (8) and the locking gear II (10) are respectively matched with the dovetailed pressing block locking pushing rod (1) and are respectively assembled on a double-end stud. The pneumatic locking and expansion fixed structure disclosed by the utility model has the beneficial effects that one cylinder controls the dovetailed pressing block locking pushing rod for controlling the pneumatic locking and expansion device for auxiliary supporting to the bearing by the upper and lower horizontal shafts in the four-side moulder, and simultaneously utilizes the connection of an opened pin and the pin shaft with the hole to avoid the phenomenon of automatic locking when the fixed structure moves.