The invention provides a method and device for detecting a polarization extinction ratio of an optical polarizer. The detection method comprises the following steps of: making linear polarized light incident along a principal axis of an optical polarizer to be detected, and obtaining two light signals on two orthogonal optical axes at an outgoing end due to polarization crosstalk; interfering and carrying out photoelectric conversion and data acquisition on interfering light signals to obtain one group of discrete voltage signal values reflecting the interfering light intensity signals; and calculating coupling intensity h at each coupling spot based on a formula (1), and calculating an extinction ratio of the optical polarizer to be detected based on a formula (2). The device sequentially comprises a light source, an isolator module, a polarization module, the optical polarizer to be detected, a projection direction regulating module, an interferometer module, a data acquisition module and a central processor module. The invention has the advantage that the coupling intensity of the coupling spots is detected to induce the polarization extinction ratio, so that the measurement result is accurate. The invention is suitable for various optical polarizers, such as polarization maintaining fibers, polarization maintaining fiber couplers, polarizers and the like and has certain universality.