Most of adaptive optics systems work in nighttime. Polarization filter technique is applied to suppress sky background for detecting the weak objects in daytime. The theory of polarization filter is introduced. Different kinds of polaroid sheets are placed between the adaptive optics system and the secondary mirror of 1.8 m telescope in Lijiang observation station. Experimental results indicate that polarization filter technique can suppress sky background light effectively and improve the detectability of adaptive optics system. It is shown that polarization filter technique can be used for observing the stellar stars in daytime.