Abstract: Ellipsometric measurement technology is a commonly used method for measuring parameters such as refractive index, extinction coefficient, and film thickness of thin film samples. An ellipsometric measurement device and method based on dual elastic optical differential frequency modulation was introduced. A measurement system based on elastic optical modulation was designed, which modulated the optical signal and loaded the ellipsometric parameter information into the optical carrier signal. The optical carrier signal was demodulated by using digital phaselocked technology to obtain the information of ellipsometric parameter, thereby achieving the measurement of thin film thickness. The sample to be tested was a circular silicon wafer coated with SiO₂ film. The spectral ellipsometer (ESS01) produced by Quantuo Company in China measures the film thickness of 105.008 nm with a standard deviation of 0.731 nm, and the designed measurement device measures the film thickness of the sample to be 105.167 nm with a measurement repeatability of 0.091. Through comparative experiments with advanced domestic instruments, it is shown that the proposed method can achieve accurate and rapid measurement of ellipsometric parameters.