A method for real-timely monitoring thickness change of a coating film is disclosed. In the method, a coating module having a chamber and a film thickness-monitoring module containing an SPR optical fiber sensor, a light source, a light-receiving detector, and optical fibers are first provided. The optical fibers are used to connect the SPR optical fiber sensor with the light source and the light-receiving detector. The SPR optical fiber sensor has a sensing area and is arranged in the chamber. The light source provides the SPR optical fiber sensor with light. Then, a substrate is put into the chamber. While coating process is performed on the substrate, a film is also formed on the sensing area of the SPR optical fiber sensor. The light-receiving detector receives signals output from the sensing area of the SPR optical fiber sensor and then outputs signals of light-intensity change.