Based on the water-absorbing and swelling properties of sodium montmorillonite and the sodium ion exchange properties of sodium montmorillonite, a new type of composite flame retardant was made from sodium montmorillonite by adsorption of urea (UM), ammonium dihydrogen phosphate (ADP), and melamine (MEL), respectively, which was coated on the surface of the paper to prepare flame-retardant paper. The microstructure, flame retardancy and mechanical properties of flame-retardant paper were analyzed by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), vertical combustion test and mechanical test. Research shows that, compared to blank paper, the flame-retardant paper obtained by coating with MMT-UM, MMT-ADP and MMT-MEL has significantly improved flame-retardant properties and enhanced thermal stability. With the increase of flame retardant dosage, the carbonization length is obviously shorter, and the mechanical properties such as elongation at break, tear strength and stiffness of the prepared flame retardant paper were improved to different degrees.