A postal envelope bag having a cushioning function includes two outer films, two inner films, heat resisting materials, transverse heat-sealing lines, longitudinal heat-sealing lines, air columns, neck heat-sealing lines, an air filling passage, air inlets and heat-sealing blocks, where air entering the air-filling port expands the air filling passage, the two outer films are pulled apart outward in a longitudinal direction and are contracted in a transverse direction; the heat-sealing blocks thrusts the two inner films, the portions of the two inner films coated with the heat resisting material are pulled apart outward in the longitudinal direction, and third segments drives the two inner films to open the air inlet as the two outer films are pulled apart outward in the longitudinal direction; after entering the air column, the air presses the second sides of the two inner films to close the air column.