Abstract: In order to overcome the problems of poor reliability and thermal sensitivity for the current optical separation system of four-frequency differential laser gyros, a digital signal demodulating system based on FPGA was presented. After analyzing the demands of signal demodulation for four-frequency ring laser gyro, the low-pass filter was designed in detail and was implemented, which was the key component of digital signal demodulating system. The pass band cutoff frequency and stop band cut-off frequency were calculated on the basis of parameters of four-frequency differential laser gyros. The parameter design of FIR filter was realized by using the FDATool of Matlab software, which is the design and analysis tool of filter. Using the FPGA integrated development environment (ISE), the proposed FIR digital filter was implemented via SOPC architecture. The simulation and experiment results show that the sum frequency component is reduced to -66 dB by designed FIR digital filter, and it meets the requirements of the digital signal demodulating system of fourfrequency differential laser gyros, and it helps to realize the miniaturization and digitization of full digital four-frequency differential laser gyros.