Polyphosphazene/SiO₂ composite membranes and porous polyphosphazene membranes were prepared and their properties were studied in detail. In this work, linear polyphosphazenes with 3-aminopropyltriethoxysilane (APTS) and n-butylamino side groups were prepared via the nucleophilic substitution reaction. The substituted polyphosphazenes were cross-linked by the hydrolysis of triethoxysilanes to form polyphosphazene/SiO₂ composite membranes. When the as-prepared polyphosphazene/SiO₂ composites were treated with HF, the silica dissolved and porous polyphosphazene membranes formed subsequently. Their structures and morphologies were studied by IR, DSA, DMA, XPS and SEM. And their mechanical properties were investigated as well. It could be seen that the thermal stability, the contact angles and the mechanical properties changed with the ratio of APTS. Namely, the contact angle changed from 74° to 96°, and the initial decomposition temperature increased from 170 °C to 225 °C when the ratio of APTS increased. Besides, the glass transition temperature increased from −48 °C to 31 °C after the membranes were cross-linked.