The invention discloses a method for preparing fluorescent adjustable p-phenylenediamine carbon dots, wherein the method comprises the following steps: 1) preparation of p-phenylenediamine carbon dots: dissolving p-phenylenediamine powder in ethanol, then sintering, separating and eluting; and 2) preparation of the fluorescent adjustable p-phenylenediamine carbon dots: adding tert-butyl peroxide radicals into an aqueous solution of the p-phenylenediamine carbon dots obtained in the step 1), and incubating, to obtain the carbon dots with emission sites at 450 nm, 490 nm and 530 nm. The invention provides the method for adding high-concentration tert-butyl peroxide radicals to change the particle size of the p-phenylenediamine carbon dots and thus obtaining the carbon dots at different fluorescence emission sites for the first time. By changing the precursor materials prepared by the carbon dots, the carbon dots with different fluorescence emission wavelengths are obtained. The operationis simple and the cost is saved.