To improve mechanical properties of ultra-low density fiberboards (ULDFs), Si-Al compounds were mixed together with fibers during preparation of ULDFs, forming a thin film on the surface of the fibers via hydrogen bonding. This work mainly optimized two proposed methods in which the inorganic thin film was assembled on the surface of fibers, in terms of its effect on the mechanical properties of fibers. Microstructural characterization (such as micromorphology and elemental distribution, chemical bonding, and crystalline phase) of Si-Al compounds and ULDFs was done to evaluate the effects. The results revealed that an inorganic thin film (probably Al2O3-SiO2) covered the surface of the fibers. Compared with the control specimen, the modulus of elasticity, modulus of rupture, and internal bond strength of the specimen treated by the sol-gel process increased from 3.87 MPa to 13.19 MPa, 0.05 MPa to 0.16 MPa, and 0.010 MPa to 0.025 MPa, respectively. Based on its higher mechanical properties, a combined sol-gel method was judged to be better for enhancement of fibers than a separate deposition method.