Square air box in kiln and self-circulation drying equipment

The utility model provides a square air box in a kiln and a self-circulation drying device, the square air box in the kiln comprises an air box bottom plate, an air box cover plate, an air box cover plate and an air box cover plate, the air box bottom plate forms the bottom of the in-kiln square air box, and the ash removal hole is communicated with the external space and the internal space of the in-kiln square air box; the rails are connected to the air box bottom plate and arranged on the two sides of the ash removal hole; and the ash removal hole insertion plate is in sliding fit with the track so as to shield or open the ash removal hole. The square air box in the kiln is provided with the track and the ash removal hole inserting plate, the ash removal hole can be shielded or opened by drawing and inserting the ash removal hole inserting plate, and due to the fact that the ash removal hole is formed in the bottom of the square air box in the kiln, dust in the square air box in the kiln directly falls dow