The invention discloses a preparation method of a modified azo yellow pigment derivative dispersant as well as a prepared derivative dispersant and application thereof. The preparation method comprises the following steps of: (1) dissolving 4-aminobenzene phosphonic acid into water, adding alkali and ketene dimer, and stirring, reacting and drying to obtain 4-acetoacetyl aminobenzene phosphonate;(2) adding 3,3-dichlorobenzidine into a hydrochloric acid solution, stirring, adding a nitrite solution and reacting to obtain a diazonium solution; (3) under a stirring state, adding acetoacetanilide and the 4-acetoacetyl aminobenzene phosphonate into an alkali solution, and dropping an acetic acid solution to obtain a coupling solution; (4) adding the diazonium solution into the coupling solution and coupling to obtain azo yellow pigment derivative color paste; and (5) adding fatty amine into the azo yellow pigment derivative color paste, and stirring, filtering, washing by water and dryingto obtain a modified azo yellow pigment derivative dispersant product. When the dispersant is used, the added quantity of the dispersant occupies 5-15 weight percent of the quantity of azo yellow pigment.