Lithium niobate on insulator (LNOI) is an ideal platform for realizing high-speed photonic integrated circuits (PICs). By taking full advantage of LN, the LNOI platform enables high-speed electro-optic and efficient nonlinear optical integrated devices. The polarization beam splitter (PBS) is one of the key passive devices for separating and combining two orthogonal polarized modes, which is essential for realizing on-chip polarization multiplexing systems and enhancing the data transmission capacity of photonic communication systems. In recent years, PBS has been successfully implemented based on various structures. Among them, the PBS based on subwavelength grating-assisted directional coupling structure stands out due to its excellent device performance and compact size. In this paper, we present a high-performance PBS with a subwavelength grating-assisted directional coupling structure, realized through an indirect etching scheme of silicon nitride-lithium niobate heterogeneous integration. Simulation results show that the device achieves a polarization extinction ratio greater than 24.49 dB in the wavelength range of 1 500 nm to 1 600 nm. Experimental data further confirms that the polarization extinction ratio of the device is greater than 18.06 dB in the wavelength range of 1 500 nm to 1 580 nm.