A 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 tube and a slider. The slider can be moved along a first bore and a second bore of the tube. The motion of the slider in the tube is similar to the motion of a piston body in a cylinder. The tube is provided with a tapered annular surface, between the first bore and the second bore, and defines an elongated opening. In addition, when the pressure of the compressed air exceeds a predetermined pressure, excessive air can be released into the ambient environment, without using a mechanical safety valve, so that an object can be prevented from over-inflation.