A connecting device of large-diameter compressed air conveying pipe comprises an outer hull and a sealing device. An annular mounting slot is set within the outer hull with two annular slopes on both sides, the sealing device includes an annular frame body, two pressing rings and two “O” sealing rings. The two pressing rings are set on both sides of the annular frame body. An internal annular slope and an external annular slope are set on inner corner and outer corner of the pressing ring respectively. Two second annular slopes are set at two tops of the pressing ring respectively. “O” sealing rings are respectively sat on the internal annular slopes of the pressing rings. Two ends of the annular frame body press the “O” sealing rings respectively.