Water hardness has an important effect on the settling characteristics of coal tailings. This article is mainly on a research of the relationship between water hardness and the settling performance of coal tailings. Samples from five coal preparation plants were used for mineralogical and water chemistry analyses by the X-ray diffraction (XRD) technique and the ion chromatograph technique, which showed that the water hardness of the recycled water was determined by the mineral composition of coal slimes. The settling tests indicated that water hardness of the sample water was closely related to mineral composition and could be used to evaluate the settling characteristics. It was shown that the increase of water hardness resulted in the increase of settling rate and the transmittance of supernatants. A gradual decrease of the slope was obtained in the curves of settling rate and transmittance. Verification tests on settling rate and transmittance of five coal samples were in agreement with the claim. The Zaozhuang and Datun samples, whose water hardness was at a high level (over 90 degrees DH), achieved the settling rate over 19 cm/min and transmittance of supernatant over 89%. Settling rate and transmittance of the supernatant are below 12 cm/min and 41%, respectively, for the Xingtai, Shitai, and Linhuan samples, whose water hardness is below 10 degrees DH.