The invention discloses a deformation detecting method and a deformation detecting system for a pantograph of an overhead contact system. Matching comparison is performed on a pantograph edge picture which is acquired in real time and a pantograph template picture. Nearest white pixels which are respectively above and below the simulated line of the pantograph slide plate are searched in a pantograph edge picture in a line-to-line manner. All searched white pixels are used as the actual lines of the slide plate. Furthermore pixel heights of all white pixels in the actual lines of the slide plate are calculated. Through comparing the pixel heights of all white pixels in the actual lines of the slide plate with the pixel heights of all points in the simulated lines of the slide plate, the deformation amount of the slide plate of the pantograph is computed. When the deformation amount is larger than a deformation threshold, a deformation defect of the pantograph is determined, and a deformation defect alarm signal is output. After the system receives the alarm signal, a corresponding alarm and a subsequent action can be triggered. Furthermore along with increase of the deformation amount, pantograph deformation abnormity increases. If a plurality of continuous frames determine the deformation defect of the pantograph, permanent deformation of the pantograph head is determined.