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.