The present invention provides a detection method, a detection system, and a detection device for flexible substrate; and the detection method comprises: obtaining the light intensity distribution by parallel light with the same intensity transmitted through the flexible substrate; finding a difference point of light intensity from the light intensity distribution; obtaining a position of the difference point of light intensity corresponding to the flexible substrate, and the position is treated as an abnormal position in the flexible substrate. Wherein the difference point of light intensity is a point where the light intensity is different from the light intensity of the surrounding area. The abnormal positions can be obtained by the detection method of the present invention.