The invention discloses a manufacturing process of antiflaming formaldehyde free density board. Under the conditions that temperature is 170-180 DEG C, the stewing pressure is 9.5+/-0.5 bar, and the stewing time is 3.5+/-0.3 min, poplar trees are softened by steam, and then are milled into wood fibers by a heat mill, the wood fibers must comprise the following three dimensions and proportions of the wood fibers that the wood fibers with length of 2 mm to 5.4 mm, length-width ratio of 63-110 and proportion of 10% to 20%, the wood fibers with length of 0.125-2 mm, length-width ratio of 35-63 andproportion of 70% to 90%, and the remaining wood fibers with length of 0.06-0.125 mm and length-width ratio of 20-35; 40+/-5 kg/m<3> of isocyanate, 5.5+/-0.5 kg/m<3> of paraffin, 57+/-4 kg/m<3> of aphosphorus-series flame retardantand 1.8+/-0.6 kg/m<3> are mixed, and then are dried to make the moisture content of the fibers must be 7+/-0.5%; and finally the mixed materials are pressed and formedafter sequentially passing through five temperature zones in the hot press. According to the density board manufactured by the process, the performances in three aspects of internal binding strength,formaldehyde emission and flame retardance are simultaneously improved.