A single-shot quantitative phase imaging (QPI) method with improved resolution based on Kramers-Kronig relations is proposed. Two pairs of in-line holograms containing the high-frequency information in the x and y directions are recorded by a polarization camera in a single exposure, which makes the recording setup compact. The deduced Kramers-Kronig relations based on multiplexing polarization can successfully separate recorded amplitude and phase information. The experimental results demonstrate that the resolution can be doubled by using the proposed method. This technique is expected to be used in the fields of biomedicine and surface inspection.