The utility model discloses an automatic monitoring system for states of a brake cylinder during a train air brake test. The automatic monitoring system comprises a mobile detection unit, a track, a rail edge control unit, a train number identification unit and a central host. The mobile detection unit is slidingly arranged on the track and is used for collecting image information about a bottom portion of a train. The rail edge control unit is used for controlling communication between the mobile detection unit and the central host. The train number identification unit is used for identifying the number of the train that enters a detection track path. The central host is used for controlling the mobile detection unit to move on the track toward the brake cylinder under a compartment and for controlling the mobile detection unit to collect images of the brake cylinder. The advantages are that the automatic monitoring system is capable of tracking and shooting parts that need to be detected on the bottom portion of the train in a fixed-point manner, so that the positioning precision is high; and the system also can shoot in a dynamic manner when the train is moving, thereby identifying faults of dynamic images, realizing automatic detection of a railway and relieving manpower.