Method for preparing black fluorane thermal-pressure sensitive dye by virtue of one-step method

The invention discloses a method for preparing a black fluorane thermal-pressure sensitive dye by virtue of a one-step method. The method comprises the following steps: (1) acid-catalysis cyclic condensation: adding a diphenyl ketone derivative and 2-methyl-4-methoxydiphenylamine into an organic solvent I with a relatively low boiling point, adding a sulfuric acid solution while stirring, and heating to a reflux temperature to react for 1.5-4.5 hours; and (2) post-processing refining: adding water into reaction liquid obtained in the step (1), recycling the organic solvent I with the relatively low boiling point by virtue of heat release, then adding a nonpolar solvent II into the recycled reaction liquid, carrying out heating reflux to separate a water phase so as to obtain an organic phase, carrying out cooling, crystallization, filtering and solvent washing on the organic phase, and drying, so as to obtain the black fluorane thermal-pressure sensitive dye.