Photonic integrated circuits (PICs) based on silicon-on-insulator (SOI) platform with the advantages of high-index-contrast and CMOS-compatible process can efficiently reduce the component sizes and densely integrate them at a chip scale. To meet the ever-increasing demand for the optical interconnect capacity, various multiplexing techniques have been used. However, it should still be proposed to effectively reduce the component size accompanied with the reasonable performance and wavelength division multiplexing (WDM) compatibility. To the best of our knowledge, there has no attempt so far to design a polarization demultiplexer based on a microring resonator in slot waveguide structures. In this paper, a compact silicon-based polarization demultiplexer is proposed, where two regular silicon-based waveguides are used as the input/output channels and a microring in slot waveguide structures is used as the polarization/wavelength-selective component. A full-vectorial finite-difference frequency-domain method is utilized to study the modal characteristics of the regular and slot silicon-based waveguides, where the effective indices and coupling for transverse magnetic (TM) and transverse electric (TE) modes are presented. With the unique modal characteristics of slot waveguides and the strong polarization-dependent features of microring resonator, we can show that the field distributions and the effective indices of the TM mode between the regular and slot waveguides are similar, while those of the TE mode show clearly different. As a result, the input TM mode outputs from the drop port at the resonant wavelength, while the input TE mode outputs from the through port directly with nearly neglected coupling, thus the two polarizations are separated efficiently. A three-dimensional finite-difference time-domain method is utilized to study the spectrum and transmission characteristics of the proposed device. From the results, a polarization demultiplexer with a radius of 3.489 m is achieved with the extinction ratio and insertion loss of ～ 26.12(36.67) dB and ～ 0.49(0.09) dB respectively for the TM(TE) mode at the wavelength of 1.55 m by carefully optimizing the key structural parameters. In addition, taking the fabrication errors into account during the practical process, the fabrication tolerances to the proposed device are analyzed in detail and the performance is assessed by the extinction ratio and insertion loss. For demonstrating the transmission characteristics of the designed polarization (de) multipexing (P-DEMUX) device, the evolution along the propagation distance of the input mode through the designed P-DEMUX is also presented. The present polarization demultiplexer is compatible with the WDM systems on-chip based on microring resonators and can be easily introduced into the WDM system to further increase the optical interconnect capacity.