The invention relates to a preparation method of a black flame retardant silk broadcloth. The preparation method comprises the following steps: step one, placing ammonium polyphosphate or melamine coated ammonium polyphosphate in a beaker, adding polyethylene glycol 400, adding water into the beaker, and carrying out ultrasonic dispersion to obtain ammonium polyphosphate turbid liquid; step two, adding deionized water into the ammonium polyphosphate turbid liquid obtained in the step one, adding dopamine to adjust the pH to 8.5-9, carrying out reactions for a while under continuous stirring, carrying out centrifugation, washing the reaction product by water for three times, then washing the reaction product by alcohol for three times, storing the reaction product in a reagent bottle, placing the reagent bottle in a baking oven, and drying the reaction product for one night to obtain polydopamine microcapsules; step three, preparing a solution from the polydopamine microcapsules obtained in the step two, adopting a buffer solution with a pH of 8.5, adding silk broadcloth, wherein the bath ratio is 1:30-50, carrying out water bath under vibration, taking out the silk broadcloth, washing the silk broadcloth by water, and drying the silk broadcloth in the air to obtain the black flame retardant silk broadcloth. The invention also relates to a black flame retardant silk broadcloth obtained by the provided preparation method. The preparation method has the advantages that the silk broadcloth is enabled to have a flame retardant property, the dyeing and finishing are integrated, the color is uniform, energy is saved, and the discharge is reduced.