The quality of clock signal plays a key role in synchronization, demultiplexing and optical decision. The extinction ratio of optical clock signal can be greatly increased after passing through birefringent fiber loop mirror (FLM), due to its flexibly tunable filtering characteristics, and then the clock signal quality is improved. We propose a method of using high-birefringence fiber loop mirror to enhance the extinction ratio (ER), numerically simulate the transmission spectrum variation of birefringent fiber loop mirror by adjusting birefringence device' polarization delay, fast/slow axis angle and optical path difference of the polarization controller; and theoretically analyze the clock spectrum after it passes through birefringent fiber loop mirror with an unperfect extinction ratio. With a 40 GHz clock signal extracted by Fabry-Pérot filter as input, an experiment is performed. The result shows that the extinction ratio of the input clock is improved by 13 dB, which verifies the results of numerical simulation.