Abstract: In this paper, MgO powder was prepared by precipitation for thermal battery. By adjusting the ratio of precipitating agent, MgO powder of different pore structure was obtained. Through X-ray diffraction(XRD), scanning electron microscopy(SEM),transmission electron microscope (TEM), nitrogen adsorption-desorption, and fourier transform infrared spectroscopy(FT-IR), the microstructure and pore structure of the powder were analyzed. The correlation between the pore structure and electrolyte leakage and electric properties of thermal battery were discussed in details. The results show that: the ratio of precipitating agent can significantly alter the pore structure; when the molar ratio of ammonia and magnesium salt is 1.5: 1, it forms a large number of mesopores.The pore volume is 0.179 cm³/g and the surface area is 30.91 m²/g.When the amount of MgO powder exceeds 35 %(mass fraction), the electrolyte leakage of the separators is only 6 mg/cm².While the electrolyte is a kind of eutectic salt (LiCl-KCl), the pore size of MgO powder should be between 2~200 nm.High electrolyte leakage leads to a shorter average working time of thermal batteries.When the pore size of magnesium oxide is too small, it will increase the average activation time of thermal batteries.