The invention discloses a printing plate tensioning air bag of a printer. The printing plate tensioning air bag of the printer can be frequently operated under a high air pressure. The printing plate tensioning air bag of the printer comprises a canvas pipe, two metal end caps installed at the two heads of the canvas pipe respectively and a sealing device. Each metal end cap comprises a metal inner core and a metal pressing ring. The sealing device comprises two O-type sealing rings and a plurality of plane sealing strips. The metal inner cores are inserted into an inner cavity of the canvas pipe and two grooves are formed in the periphery of each metal inner core; each metal pressing ring is arranged on the periphery of the canvas pipe in a sleeved mode and is composed of an upper half pressing ring and a lower half pressing ring and each upper half pressing ring and the corresponding lower half pressing ring are connected through a plurality of screws, the combined faces of the upper half pressing ring/lower half pressing ring and the lower half pressing ring/lower half pressing ring are correspondingly provided with a plurality of maze grooves and a plurality of protrusions respectively; the two O-type sealing rings are embedded into the two grooves formed in each metal inner core correspondingly; the plane sealing strips are embedded in the portion between the top faces of the protrusions of the upper half pressing ring/lower half pressing ring and the bottom faces of the maize grooves formed in the lower half pressing ring/upper half pressing ring.