The feasibility and technological conditions of the preparation of flame retardant paper based on fiber using co-precipitation synthesis of Mg-Al hydrotalcites as filler in papers were investigated. The crystallizations, granularities of Mg-Al hydrotalcites and characters of flame retardant papers were investigated through transmission election microscope and X-ray diffraction technique etc. The results indicated that Mg-Al hydrotalcites were layered hexagonal nanoparticles, with perfectly crystallized structure and striking performance in furnish retention improvement. The flame retardant papers based on fiber using Mg-Al hydrotalcites as fillers have excellent inflaming retarding performance. The oxygen index of the flame retardant paper produced is above 25 % at the dosage of 15 wt%.