This paper describes the synthesis and properties of the porous MgO fibers used as a binder of the electrolyte for thermal batteries. Porous MgO fibers with controlled pore size were prepared by a facile hydrothermal method in the presence of polyethylene glycol (PEG). The effect of PEG addition on the pore sizes of the porous MgO fibers was systematically investigated. The influence of porous MgO fibers having different pore sizes on the electrolyte (LiCl-KCl) leakage and electrochemical performances of the corresponding model cells were measured. The results show that a large amount of PEG leads to porous MgO fibers having high specific surface area and total pore volume, and reaching the maximum, 13.65m2 g−1 and 0.097m3 g−1 respectively, at 1.5mol% of PEG addition. The model cell fabricated with porous MgO fiber having high surface area and pore volume shows low deformation and electrolyte leakage, as well as high discharge capacity. Electrochemical impedance spectroscopy (EIS) analysis shows that, the ionic conductivity of the cell increases as the porous MgO fiber pore size increasing. These performances are much better than the performances of cell fabricated with traditional MgO powders. The dimensional stability and large surface area contact of porous MgO fibers with molten electrolyte should account for the superior physical and electrochemical performances of model cell fabricated with porous MgO fibers.