The invention discloses a pipeline air pressure on-line detector, comprising detection parts, a hand valve and an air inlet pipe. The detection parts comprise a piston housing, a piston and a piston rod, wherein one end of the housing is provided with a chamber cover, one end of the piston rod passes through the chamber cover, the piston rod is provided with a scale and both sides of the piston are respectively provided with a detection spring. The invention also discloses another pipeline air pressure on-line detection method. Under the conditions that the air inlet pipe communicates with the pipeline to be detected and the hand valve is opened, the piston moves under the effect of pipeline air pressure and the piston rod is driven to stretch. The air pressure state value of the pipeline can be detected through the reading of the scale. Therefore, the measuring apparatus has advantages such as high measurement accuracy, on-line detection option, positive and negative air pressure detection option, simple and convenient operation, high efficiency and the like.