<p>Boron mud, a byproduct of borax production, contains significant magnesium resources that can be utilized to prepare magnesium oxysulfate plates through further processing of magnesium oxysulfate cement. In this study, roasted boron mud was used as a raw material to prepare standard magnesium oxysulfate plates. The effects of industrial-grade light-burned MgO activity, modifier selection, fiber type, and dosage on flexural strength were investigated using an electronic universal testing machine. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to analyze the phase composition and microstructure of the magnesium oxysulfate plates. The results indicate that when the activity of light-burned MgO was 60%, the modifier was a mixture of phosphoric acid and citric acid, and the basalt fiber content was 4%, the 5·1·7 phase (5&#xa0;Mg(OH)₂·MgSO₄·7H₂O) in the magnesium oxysulfate plate was maximized after 28&#xa0;days of curing, achieving a flexural strength of 15.1&#xa0;MPa. At this condition, the plate exhibited a volume density of 1.56&#xa0;g/cm³, a softening coefficient of 0.9, a moisture content of 5%, a shrinkage rate of 0.2%, and a wet swelling rate of 0.2%. These properties meet the F-grade requirements of the industry standard "Magnesia Cement Products—Magnesium Sulphide Plate" (T/CMMA 8–2020). This study provides a theoretical basis for the value-added utilization of boron mud and a new method for preparing magnesium oxysulfate plates.</p> Graphical Abstract <p></p>