Polymer-based structural membranes have been considered as highly-efficient passive radiative cooling (PRC) materials but limited by the hazardous process and insufficient solar reflectivity during daytime. This work reported a sustainable phase-separation strategy for the fabrication of 3D porous polydimethylsiloxane/ZnO (3D PDMS/ZnO) hybrid membrane. The water was used as emulsifier of PDMS and as the evaporation of water drops in sealed environment, uniform cavities within 10 μm self-formed automatically, which were demonstrated to reflect more than 96% of solar radiations by cavities scattering. Meanwhile, the hybrid structure imparted the membrane with highly thermal emissivity surpassing 80% calculated from the transmittance. As the proof concept, the prepared film was used for outdoor daytime radiative cooling and presented excellent cooling performance, which could achieve about 7.5 °C of cooling effect even under the high solar radiation (∼906 W/m²). Moreover, the membrane showed excellent flame tolerance which satisfied the requirement for thermal safety of thermal-application materials. Therefore, the sustainably structural strategy and hybrid structure would propel the development of highly energy-efficient and thermal safe PRC materials.