It was proved that MgO and MicroSilica can react at room temperature, giving a hardened product primarily comprised of Mg(OH)2 and Magnesium Silicate Hydrate (M-S-H). The reaction ratio and process and the chemical composion of M-S-H were studied and analyzed by QXRD and DTA-TG. The experimental results indicate that much Mg(OH)2 and less M-S-H were formed at early period. After 7 days there is no change in the quantity of Mg(OH)2, while M-S-H was increased slowly. The chemical composion of M-S-H would vary with the mix proportion in the hydration process , but M1.32SH2.37 is finally the approximate form.