A superhydrophobic porous film was fabricated by a simple phase separation process between a polyhedral oligomeric silsesquioxane (POSS) based diblock polymer (P) and polyethylene glycol (PEG) in selective solvents. The POSS-based hybrid diblock polymer was synthesized using a two-step atom transfer radical polymerization method. The polymer, along with PEG, was dissolved in mixed solvents of tetrahydrofuran (THF) and ethanol (AC), and then dip-coated onto the glass slides to form a coating film. After PEG was washed away using water, the polymer film with nano-micrometer dual-scale roughness on the slides was formed due to the phase separation. Then, a superhydrophobic porous film was obtained after surface modification with low surface energy material. The effects of POSS content, P/PEG ratio, and AC/THF ratio were investigated. The results showed that the water contact angle increased with the PEG content within a certain range and the surface roughness was enhanced after adding AC. When the POSS content was 25.3% (P2), the ratio of P2/PEG was 5:5 and AC/THF was 3:7, the film after modification had a static water contact angle of 155.1 degrees and a sliding angle of 6.5 degrees, demonstrating superhydrophobicity.