A method for measuring the thickness of photoresist, namely two-spectrum method, is proposed. Using AZ4620 photoresist throw at plane glass, the measuring thickness of photoresist is benchmark through ellipsometer. Compared with incident lights, the intensity change of emitted lights is examined by means of two spectrum method measuring. The purpose of measuring the thickness of photoresist is achieved. The result deviation is less than 2 percent. Compared with traditional methods for measuring the thickness of film, the new method has the advantages of simple computation and good maneuverability. As photoresist has the exposal characteristics, two-spectrum method is fit for measuring the thickness of photoresist.