The space group and unit cell of yingjiangite from the Xiazhuasng ura- nium deposit, Guangdong Province, have been determined for the first time by single-crystal X-ray diffraction analysis.Its unit cell is orthorhombic, space group Bmmb (63)，a=15.707 (3)，b=17.424(3)，c=13.692(2)A. V = 3747A3.Z=4.Dc二4.60g/cm3, Dm=4.54 g/cm3.Chemical formula: (K2,Ca) (UO2)7(P04)4(OH)6·6 H2O.Biaxial negative with 2Vc=36°，2 Vm=36-38°， Ng=1.707(2)，Nm=1.703(2)，Np=1.666(1)·Optical oiientatiori: , Ng//Y, Nm//X and Np//Z.