We report the development of double-cross-linked thin film nanocomposite (TFN) hollow fiber membranes based on polyhedral oligomeric silsesquioxane (POSS) at scalable fabrication conditions for ultrafast brackish water desalination. The optimized spinning conditions allowed the fabrication of effective and robust hollow fiber supports at high spinning speed, zero water content in the dope, and relatively large inner dimension. POSS was then introduced into the interfacial polymerization process to form a double cross-linked network. With proper pH adjustment, the water permeability was enhanced by 47.7% with higher salt rejection, eventually reaching a pure water permeability of 7.5 +/- 0.2 L m(-2) h(-1) bar(-1) and 98.3 +/- 0.2% rejection to NaCl after post-treatment. The mechanism was revealed by different techniques such as positron annihilation spectra and zeta potential. The membrane also demonstrated resistance to bacteria attachment and showed promising results in treating membrane bioreactor filtrate from a local water reclamation plant.