The present invention provides a film edge detecting method and a film edge detecting device. The film edge detecting method is used for detecting a film edge of a film layer formed on a substrate, the film layer comprises a patterned film layer, the method includes: forming at least one scale pattern in the patterned film layer, a film edge of the patterned film layer corresponding to an edge of the scale pattern; obtaining a patterned film edge indication value of the edge of the scale pattern; and obtaining a second distance, which is a distance between the film edge of the non-patterned film layer and a corresponding edge of the substrate, based on the non-patterned film edge indication value and a preset reference value of the corresponding edge of the substrate.