To clarify how the mechanical properties of ultra-low density fiberboards (ULDFs) affected by Si-Al molar ratios, they were prepared with different Si-Al molar ratios. Microstructure and mechanical properties of the ULDFs were tested using scanning electron microscope, energy dispersive spectroscopy, X-ray photoelectron spectrometer, Fourier transform infrared spectrometer, X-ray diffractometer, and microcomputer control electronic universal testing machine. The results showed that Si and Al component were uniformly distributed on the fibers’ surface and the bond of Si-O-C was formed. The different microstructures and relative densities were presented with different Si-Al molar ratios. The results of the modulus of elasticity (MOE), modulus of rupture (MOR) and internal bond strength (IB) were also significantly affected by different Si-Al molar ratios; and their maximum values of 20.78, 0.17, and 0.025MPa were obtained while Si-Al compounds with Si-Al molar ratio of 2:1 was added.