The invention provides an air tightness detecting device and detecting method for a movement process of a high speed train. The device includes two air pressure transmitters and one single chip microprocessor disposed on a head locomotive of the high speed train and two air pressure transmitters and one single chip microprocessor disposed on a tail locomotive of the high speed train. The device isadvantaged in that the structure is simple and air pressure leakage information can be displayed in real time. When air pressure leakage occurs, the position of an air pressure leakage point can be calculated; data can be stored in a CCU(Central Control Unit) and can be transmitted to a DMI (Driver-Machine Interface) through an MVB (Multifunction Vehicle Bus) connected with the CCU. The detectionmethod is that when air pressure leakage occurs, the single chip microprocessors on the head locomotive and the tail locomotive send air pressure abnormal values and abnormal moments measured by theair pressure transmitters to the CCU, calculate the air pressure leakage position according to time difference in combination with vehicle speed, vehicle length and air pressure propagation rate and gives real time alarm and display in the DMI. At the same time, the CCU can be started during in-factory maintenance of the high speed train and information of the air pressure leakage position can becalled, so that a large amount of time and labor can be saved in air tightness check.