In this paper, the coordination compound ［Mg(H₂O)₆］(TNR-)₂·2H₂O is prepared by the reaction between styphnic acid and magnesia. Its molecular structure and crystal structure are determined by X-ray diffraction. The crystalline is in triclinic with space group Pī. The crystal parameters of the title compound are as follows: a＝0.788 9(1) nm，b＝0.882 1(2) nm，c＝1.043 3(2) nm； α＝77.47(1)°，β＝70.96(2)°，γ＝66.74(1)°； V＝0.627 3(2) nm³，Z＝1，D_C＝1.738 g·cm^-3，μ＝0.193 mm^-1，F(000)＝338.