Abstract: Flame retardant reed fiberboard (RFB) was prepared from Baiyangdian reed fiber by using methylene diphenyl diisocyanate(MDI) as adhesive and magnesium-aluminum hydroxide (MATH) and boric acid (BA)as flame retardant agents. When the addition amount of MATH was 20% (caculated by the mass of reed, similarly here in after), flame retardant properties of RFB increased but mechanical properties reduced. After replacing parts of MATH with BA, the flame retardant properties and mechanical properties of RFB both improved. The vertical combustion grade reached V-0 and the static bending strength improved by 2.8-8.8 MPa. When 4% BA and 16% MATH was added, the time for ignition of RFB extended from 34 s to 50 s, the peak heat release rate (PHRR) reduced by 68.5% (from 290.3 kW/m² to 177.1 kW/m²), and the total smoke release (TSR) reduced by 68.6% (from 238.0 m²/m² to 119.4 m²/m²). Meanwhile the modulus of rupture of RFB increased from 9.0 MPa to 16.5 MPa. SEM, TG and FTIR analysis showed that the addition of BA promoted the solidification of MDI, and the amount of residual carbon of RFB increased significantly.