A pen-type pressure indicator, which can be detachably connected to an air compressor for measuring the pressure of compressed air produced in the air compressor, includes a transparent tube and a slider. The slider can be moved along a first bore and a second bore of the transparent tube. The motion of the slider in the transparent tube is similar to the motion of a piston in a cylinder. The transparent tube is provided with a tapered annular surface, between the first bore and the second bore. In addition to measuring the pressure of the compressed air delivered to an object, when the pressure of the compressed air exceeds a predetermined pressure, excessive air can be released into the environment, without requiring the air compressor to be installed with a safety valve, so that the object can be protected from damages due to overpressure.