The simultaneous improvement of dielectric, mechanical and water resistance properties of dielectric materials is extremely important for their practical use, but rarely been realized. Here, the comprehensive properties of polyimide (PI) film were enhanced through the construction of sandwich-type porous film using the microemulsion method. Due to the introduction of porous structure on both sides of PI film, apparent decreases were observed in dielectric constant (16.50%) and water absorption (82.7%), accompanied by increases in tensile strength (5.14%) and tensile modulus (41.17%), compared to flat PI film. Furthermore, excellent durability of low dielectric constant of the sandwich-type porous film under moist environment was demonstrated since 1.76% of increase was found upon exposing to 75% R.H. for 32h. This provides a novel method to optimize the comprehensive properties of dielectric materials and also displays underlying insight for the development of functional materials in electronic and electric areas.