Single crystal structures of M2Ni (SO4)2.XH2O (M=Na, X=4; M=K, NH4, Rb, Cs, X=6) were investigated using the single crystal X-ray diffraction method. They crystallize isotypic (space group P2 (1)/c). Many structure features exhibit regularity with the relative size of radius, such as a, b, c, V. But to ANSH, the regularity of average bond lengths of violets from others. Their optical transmission spectrums were recorded on PE-lambda 900 with performing wavelengths ranging from 200 to 1100 nm, they have the similar optical characters in tested wavelength range. This phenomenon arises from the electron transition of d orbits in the octahedral field. The similarity in optical spectrum is consistent with the comparability of structures. We also investigated the thermal-gravimetric quality of above single crystals under same experiment conditions. We found that the dehydration temperatures increase with increasing of radius of the M+ when M =K, Rb, Cs. When M=Na, the dehydration temperature (tdh) is above the others pronouncedly due to the specialty of structure. To NH4-, the so-called compound is also with exception of the regularity.