Thin-film lithium niobate (LN) has emerged as a promising platform for building chip-scale optical functional devices due to its versatile advantageous properties. A polarizer with a high extinction ratio is essential to suppress polarization crosstalk and enhance signal-to-noise ratio. Here, we demonstrate an LN-based polarizer that effectively addresses the long-standing issue of polarization crosstalk while simultaneously achieving high extinction ratio and broad bandwidth. Leveraging the pronounced loss disparity between the modes with different polarizations enhanced by resonant cavities formed by metal strips, we obtain a greatly improved polarization extinction ratio (PER) up to 42 dB, while the potential crosstalk between adjacent waveguides can be effectively suppressed down to −38 dB over a 160-nm bandwidth, with a low insertion loss (IL) of 0.7 dB at the wavelength of 1550 nm. This work paves the way for developing high-performance polarization-dependent photonic devices such as optical modulators and gyroscopes.