AbstractB-Zn-Si-Al compounds are modified Si-Al compounds made of sodium silicate, zinc sulfate, borax, and aluminum sulfate. They act as a fire retardant during the preparation of ultra-low density fiberboards (ULDFs). An orthogonal experiment was used to optimize the contents of the four compounds according to the limiting oxygen index. Fourier transform infrared spectroscopy was employed to preliminarily analyze the chemical structure of the compounds. Additionally, a cone calorimeter was used to assess the fire performance of the ULDFs. The results suggested that the optimized solution was made of 27.0 mL of sodium silicate solution, 27.0 mL of aluminum sulfate solution, 3.0 g of zinc sulfate, and 6.5 g of borax, resulting in an oxygen index of 29.5%. The bond Al-O-B was detected at 1397 and 796 cm-1. The Si-O-Zn bonds were detected at 867 cm-1. The heat release rate, total heat release, and total smoke release decreased and the mass residual ratio increased in the sample using the optimal conditions from the orthogonal array design. It was concluded that B-Zn-Si-Al compounds effectively protect fibers from fire.