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 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.