A novel terahertz polarization beam splitter (PBS) with low loss and large bandwidth based on double core negative curvature fiber is designed. The device takes copolymers of cycloolefin as the substrate, and 12 circular tubes with embedded tubes are evenly distributed along the circumference. The fiber core is divided into two cores through two groups of circumscribed small clad tubes symmetrical up and down. The finite-difference time-domain (FDTD) method is used to analyze its guide mode properties. The effects of various structural parameters on its beam splitting characteristics are investigated in detail, and the extinction ratio (ER), bandwidth and transmission loss of the PBS are analyzed. The simulation results show that when the incident light frequency is 1THz and the beam splitter length is 6.224 cm, the ER of x-polarized light reaches 120.8 dB, the bandwidth with ER above 20 dB is 0.024 THz, the ER of y-polarized light reaches 63.74 dB, the bandwidth with ER above 20 dB is 0.02THz, and the total transmission loss is as low as 0.037 dB/cm. Tolerance analysis shows that the PBS can still maintain good performance under the ±1% deviation of structural parameters.