Water-bearing cuspidine, a very rare mineral, was discovered by the authors in calcic skarn of the exocontact zone between Late Yanshanian granite and Triassic limestone in the Gejiu tin deposit, Yunnan Province. Its mineralogical characteristics, paragenesis, chemical composition, optical features, X-ray powder diffraction data, infrared absorption spectroscopy and thermal analysis were studied systematically. Associated with andradite, B_vesuvianite, diopside, datolite, fluorite, wollastonite and nordenskioldine, the mineral is characterized by clear polysynthetic twin, strong dispersion of refractive index and optic axial angle. Its optical properties areNg=1.604,Nm=1.595,Np=1.592,Ng-Np=0.012, (+)2V=62°, c∧Ng=6.5°. Electron microprobe and chemical analyses of the mineral give SiO230. 64%, TiO2 0.04%, Al2O30.01%, Cr2O30.06%, FeO 0.02%, CaO 55.61%, MgO 0.09%, MnO 0.14%, Na2O 0.02%, H2O+4.55%, F 8.63%, B2O30.59%, and CO21.43%, with the calculated empirical formula being (Ca3.979Mg0.009Mn0.008Na0.003Fe2+0.001)4(Si2.046B0.068 Cr0.003Ti0.002Al0.001)2.120O7(F1.823OH0.322)2.145(CO3)0.130·0.854 H2O. The unit cell parameters area0=10.94°A,b0=10.50°A,c0=7.623°A,β=110.42°. Infrared absorption spectral analysis suggests that there exists stretch_back vibration of OH at3560cm-1. Thermogravimetric analysis shows that the loss for H2O is about 2.8% and about1.6% for OH-. The authors consider that water-bearing cuspidine was formed in the hypabyssal exoskarn zone, being a product of contact metasomatism under the active role of F(B)-rich fluids. It was for the first time that water_bearing cuspidine was discovered in China.