The invention relates to a preparation method of an MF (melamine-formaldehyde) resin-coated Mg/Al-LDHs (magnesium/aluminum-layered double hydroxide) microcapsule. The preparation method comprises the following steps of firstly mixing melamine and formaldehyde according to a molar ratio of 1 to (2 to 4), so as to prepare a clear and transparent MF prepolymer; then, using deionized water as a dispersing medium, and preparing a flame retardant agent Mg/Al-LDHs into a core emulsion under the action of an emulsifying agent; using an in-situ polymerizing method to coat the MF prepolymer around capsule core emulsion droplets, adjusting the pH (potential of hydrogen) value of a solution to weak acid, and enabling the MF prepolymer to further complete condensation reaction at the surface of the Mg/Al-LDHs, so as to generate an insoluble latticed crosslinking structure, and form a capsule wall with certain thickness and strength; finally, using absolute ethyl alcohol to wash for multiple times, drying at the temperature of 60 DEG C, and grinding, so as to obtain the MF-coated Mg/Al-LDHs microcapsule. The preparation method has the advantages that the synergistic flame-retardant function of the nitrogen flame-retardant agent and the Mg/Al-LDHs is fully realized, and the problem of poor dispersivity and compatibility of the flame-retardant agent in a substrate is solved; meanwhile, the preparation technology is simple, and the cost is low.