Magnesium silicate hydrate (MgO-SiO2-H2O M-S-H) is a main bonding phase for microsilica bonded magnesium- castables used for tundish. Its content determines the mechanical properties after demolding. In this paper, M-S-H was synthesized with Na2SiO3·9H2O and MgCl2·6H2O as raw materials. The effect of M-S-H at different calcination parameters was investigated by X-ray diffraction, scanning electron microscopy, specific surface area measurement, nuclear magnetic resonance, and thermogravimetry-differential scanning calorimetry. Moreover, the synthesized M-S-H with microsilica powder (1%) was introduced into the magnesia-based castables to evaluate their influences on the properties. The results show that M-S-H has a memory effect when the calcination temperature is below 400 ℃. Increasing the calcination time can facilitate the insertion of interlayer hydroxyl groups in M-S-H gels and contribute to the formation of a lamellar structure with a higher structure stability. A certain amount of pre-synthesized M-S-H and a small amount of silica powder can assure the castable with a good workability and a sufficient early bonding strength. The reduced silica can suppress the formation of large sized defects due to the volume shrinkage of matrix, in the interface of aggregates and matrix, thus improving the mechanical properties of castables. This work provides a support for the development of magnesia-based castables with a low silica content.