The creation of porous materials with both good thermal insulation and flame retardancy is of great importance for construction of energy-saving coatings in many applications. Herein, we report the facile fabrication of octasilsesquioxane (POSS)-based monolithic nanoporous polymers (named as PDVB-POSS) by solvothermal method using octavinyl-POSS as monomer and divinylbenzene (DVB) as crosslinker followed by freeze drying. The as-prepared PDVB-POSS show abundant porosity with mesopore sizes ranging from 8 nm to 13 nm, which results in a high thermal insulation with a low thermal conductivity of 0.024 W m−1 k−1 in air. The PDVB-POSS also possesses excellent flame retardancy with a peak heat released rate (pHRR) of as low as 144 W g−1. Torch burn test implies that the PDVB-POSS shows self-extinguishing behaviors without generation of any melt dripping, further reflecting an excellent flame retardancy. Taking advantages of simple fabrication process, easily to be scaled-up, excellent thermal insulation and flame retardancy, such POSS-based monolithic porous polymers may have great potential as energy-saving coatings for real applications.