The invention discloses a preparation method for a polydopamine-coated red phosphorus microcapsule fire retardant. The preparation method comprises the following steps: (1) mixing a disodium hydrogenphosphate solution with a sodium dihydrogen phosphate solution to prepare a buffer solution; (2) adding a red phosphorus fire retardant, dopamine and oleic acid rare earth into the buffer solution, and then successively carrying out magnetic stirring and vacuum pumping filtration so as to obtain a layer of polydopamine-coated red phosphorus; and (3) repeating the above steps to obtain three layersof polydopamine-coated red phosphorus, and successively carrying out drying, grinding and sieving with a 200-mesh sieve so as to obtain the polydopamine-coated red phosphorus microcapsule fire retardant. According to the invention, polydopamine is adopted for three-layer coating of the red phosphorus fire retardant, so the water resistance of the red phosphorus fire retardant can be effectively improved and the compatibility of the red phosphorus fire retardant with a polymer is enhanced, and thus, thereby various mechanical properties of fire retardant resin are ensured; at the same time, polydopamine and the red phosphorus fire retardant can also produce phosphorus-nitrogen synergistic fire retardation action, so more excellent flame retardancy and thermal stability are obtained.