The invention discloses an online detection device and method for the offset of a center line of a city rail train pantograph. The online detection device comprises a central processing unit, a wheelaxle position sensor, a train number recognition antenna, photoelectric sensors, industrial flashlights, industrial cameras and a bracket. The online detection method comprises the steps that the industrial cameras acquires images of the front and rear side surfaces of the pantograph, the train number recognition antenna acquires train number information of a train detected at present; electromagnetic interference filtering processing is performed on the acquired images of the front and rear side surface of the pantograph; then edge extraction is performed to obtain the edge of the pantographand the edge of a contact line; the coordinates of a contact point are acquired by adopting a gradient direction based discrete Hough transformation algorithm and a template matching algorithm; steering knuckle positioning is performed, and boundary points of the steering knuckle are extracted to obtain the position of the center line of the pantograph; internal parameters and external parametersof the industrial cameras are obtained through a method of camera calibration, and thus the offset of the center line under an actual coordinate system is calculated. The online detection device and method adopt online non-contact measurement and have the advantages of high measurement accuracy and simple and convenient implementation.