Aiming at the problem of limited speed measurement of moving targets in a large dynamic range， a four-light coherent mixing detection method is proposed. By extracting the Doppler frequency and the difference frequency between frequency shift and Doppler frequency， the speed measurement with a larger dynamic range can be achieved. In the process of four-optical coherent mixing detection， there are many output signals and the problem of difficult signal discrimination. In order to solve the above problems， a four-light coherent mixing detection structure with spatial polarization splitting structure is proposed and designed. Polarization splitting technology is used to achieve complete spatial separation of four output signals， avoiding mutual influence between signals and reducing the difficulty of signal processing. In theory， the structure is analyzed in principle， and the optical simulation software is used to simulate and analyze it to verify the feasibility of the theory and structure. At the same time， the direction of the target movement can be judged according to the output signals of different detectors.