The dimensional and property stability after heating is considered to be one of the most important properties for low-dielectric-constant materials. Herein, porous structure contained composite PI film with porous PI structure on the surface of a flat PI film was prepared through the simple and low-cost microemulsion method. The introduction of porous coating can significantly improve the dielectric and water resistance properties of PI film while maintaining its good mechanical properties. More importantly, the porous structures were well preserved after heating, but with the remove of surfactant layer on the inner surface of the pores, leading to lower dielectric constants with 1.06%–3.50% of reduction and lower dielectric losses less than 0.015 with the maintainence of good water resistance and mechanical properties. Excellent durability of the low dielectric constants and low dielectric losses upon high humidity were also shown for the heated composite PI film. In addition, the morphology and properties of the composite PI film both before and after heating could be controlled through varying the fabrication conditions. These results provide valuable information for the construction of the thermally stable low-dielectric-constant material.