For the needs of high precision, high repeatability and fast real-time ellipsometry measurement, a stable ellipsometry measurement technique based on double elastic ellipsometry parameters was studied. The principle of the elastic modulation ellipsometry parameter measurement system was introduced, and a stable ellipsometry parameter measurement system was designed. The multi-rate processing method (downsampling method) could achieve the effect of narrowband low-pass filtering while reducing the utilization of field programmable gate array (FPGA) hardware resources. The CIC and FIR filters used in downsampling were designed, the digital phase-lock technique was used to demodulate the double-elastic modulated optical carrier signal, and the initial ellipsometry parameters were obtained. The experimental system was designed and the calibrated Si-based SiO₂ standard sample was tested for 2 000 times. The experimental results show that when the sampling time is set to 20 ms, the repeatability accuracy of amplitude ratio and phase difference is better than 0.001°, and the repeatability accuracy of film thickness of the measured sample is 0.001 nm, which verifies that the design has better measurement repeatability and higher measurement accuracy.