This article introduces a method of achieving high polarization extinction ratio using a subwavelength grating structure on a lithium niobate thin film platform, and the chip is formed on the surface of the lithium niobate thin film. The chip, with a length of just 20 micrometers, achieved a measured polarization extinction ratio of 29dB at a 1550nm wavelength. This progress not only proves the possibility of achieving a high extinction ratio on a lithium niobate thin film platform, but also offers important technical references for future work on polarization beam splitters, integrated fiber optic gyroscopes, and beyond.