The invention discloses a slay direct connection type pneumatic trimming device. The slay direct connection type pneumatic trimming device includes a weft beating-up slay, an air cylinder, a pneumatic hemmer, and a weft yarn scissors mechanism; the air cylinder is connected to the weft beating-up slay; the pneumatic hemmer is arranged on the air cylinder; the weft yarn scissors mechanism is arranged on the pneumatic hemmer; and the air cylinder, the pneumatic hemmer, and the weft yarn scissors mechanism swing with the weft beating-up slay. The air cylinder, the weft yarn scissors mechanism, and the pneumatic hemmer are integrally arranged on an air cylinder positioning base to form a unit, the air cylinder positioning base is connected to a T-shaped groove of the weft beating-up slay, and then the air cylinder, the weft yarn scissors mechanism, and the pneumatic hemmer can swing with the weft beating-up slay; and the convenience of a weft beating-up operation is improved, abrasion is reduced, and the service life is prolonged.