In order to realize the alternate detection of the in-phase and quadrature components of the signal, a coherent detection scheme based on a preset frequency offset is proposed in this paper. The hardware quantity required by this scheme is the same as that of heterodyne detection, which is half of that of homodyne detection. Relevant experiments and simulation results show that, compared with the heterodyne detection scheme. This scheme does not require a subsequent down-conversion process and has a lower bandwidth requirement for the receiver. In order to solve the signal distortion caused by dispersion, laser frequency offset and receiver bandwidth limitation in this scheme, an adaptive real-value equalization structure is proposed. It provides a low-cost and low-complexity solution for the application of coherent detection in cost-sensitive access networks.