Broader spectra, lower reflectivity and higher reliability are the performance requirements for broadband antireflective (BBAR) films. In this work, a BBAR film structure was proposed, which maintains extremely low reflectivity, ultra-wide spectra, low polarization sensitivity and practical reliability. The BBAR film consists of a dense multilayer interference stack on the bottom and a nano-grass-like alumina (NGLA) layer with a gradient low refractive index distribution on the top. The film was deposited by atomic layer deposition, while the NGLA layer was formed by means of a hot water bath on Al2O3 layer. The top NGLA layer has extremely high porosity and ultra-low refractive index, along with extremely fragile structure. To surmount the fragility of NGLA layer, a sub-nano layer of SiO2 was grown by atomic layer deposition to solidify its structure and also to adjust the refractive index with different thicknesses of SiO2. Finally, in the wide wavelength range of 400-1100 nm, the average transmittance of the double-sided coated fused quartz reaches 99.2%. The absorption, light scattering, reliability and polarization characteristics of BBAR films were investigated. An optimized BBAR film with low polarization-sensitivity and improved reliability was realized, which should be potentially promising for application in optical systems.