Bischofite is a very promising magnesium salt resource with abundant reserves and low cost. Magnesium silicate hydrate (M-S-H) gels with different MgO/SiO2 molar ratios (0.5∶1, 1∶1, 1.5∶1) were synthesized using bischofite from Qinghai Salt Lake and sodium silicate. The synthesis mechanism and structural characteristics of M-S-H were studied by XRD, SEM, infrared, and nuclear magnetic resonance tests. Then, the synthesized M-S-H was compounded with microsilica to prepare magnesia castable, and the influence of M-S-H structure on the bonding characteristics of castable was explored. The results show that M-S-H obtained from different MgO/SiO2 molar ratios exhibits a layered stacking structure. When the MgO/SiO2 molar ratio is 1∶1, the interlaminar free water is the least and the crystallinity is the highest. The synthesized M-S-H is used to replace part of microsilica to prepare magnesia castable, which significantly improves the mechanical properties of the castable after heat treatment at 1 550 ℃. The magnesia castable prepared with M-S-H (MgO/SiO2 molar ratio of 1∶1) and microsilica obtains the best comprehensive properties. Compared with the magnesia castable prepared by adding 6% (mass fraction) microsilica, the flexural strength at room temperature and high temperature increases by 75% and 8%, respectively.