The effects of single adding and mixing polyoxymethylene (POM) fibers with different lengths on fluidity, flexural strength, compressive strength, flexural toughness and drying shrinkage of mortar were studied in this paper, and the microstructure of mortar was observed by scanning electron microscope. The results show that mortar fluidity decreases with the increase of POM fiber length and content, and the influence of mixed fiber with different lengths on the fluidity of mortar is less than that of single fiber. POM fiber effectively enhances the flexural strength of mortar, but the strengthening effect is weakened when the content is more than 0.6% (volume fraction, the same below). Compared with the sample without fiber, the 6 mm fiber with 0.6% content has the highest increase of 28 d flexural strength, which is 14.67%. The compressive strength decreases with the increase of fiber content. The flexural toughness of 12 mm fiber is improved more significantly than that of 6 mm fiber and mixed fiber, which increases of 49.43% at most compared with the sample without fiber. The addition of fiber greatly reduces the drying shrinkage of the sample, the 90 d drying shrinkage of the sample decreases first and then increases with the increase of fiber content. Compared with the sample without fiber, the 90 d drying shrinkage rate of 6 mm fiber sample with 0.6% content decreases the most, which is 27.39%. The hybrid POM fiber still significantly improves the flexural strength and reduces the drying shrinkage when the content is more than 0.6%.