A compact silicon-based polarization beam splitter (PBS) using directional couplers assisted with subwavelength grating (SWG) is proposed and analyzed in detail, where a conventional directional coupler at the input/output end and an SWG coupler in the middle together with SWG-based transitions between them are used. The introduced SWG structures can significantly improve the coupling strength for TE mode, whereas that for TM mode is hardly affected. Subsequently, by carefully choosing structural dimensions, the total coupling length for TM mode can be exactly twice that of TE mode, thus the two polarization states can be effectively separated so that a PBS is realized. Results show that a PBS with a coupling length of 10  μm is achieved, and the extinction ratio, insertion loss, and reflection loss are 23.38/31.78, 0.136/0.104, and −22.03  /    −  26.91  dB, respectively, for TE/TM mode. In addition, fabrication tolerance to the key structural parameters and field evolution through the present device are both presented.