Abstract: To enhance radar resolution capability for closely spaced targets and to exploit the information advantage of the polarization dimension, this paper starts from the polarimetric radar signal model and proposes a polarization modulation spectral method based on the polarization modulation theory. By optimizing the virtual transmitted polarization, the polarization difference between targets is enlarged, thereby enabling effective estimation of the parameters of closely spaced targets. Theoretical derivation reveals the key factors affecting resolution performance. Simulation results show that the target signal-to-clutter-plus-noise ratio and the polarization difference between targets are the main factors determining resolution performance. The field experiment using fully polarimetric range-Doppler data of an anchored research vessel shows that, with 96 pulses, the proposed method improves the peak-valley separation of adjacent scattering components within the extended vessel target and exhibits a separation trend close to that obtained by conventional processing with 256 pulses at representative range cells. These results verify the feasibility of polarization modulation spectrum and virtual polarization optimization for radar resolution enhancement.