The invention discloses a preparation method of acid yellow N-R. The method comprises the following steps: (S1) introducing a benzidine compound with a structural general formula shown in the specification and an acid reagent into a reactor, and stirring and mixing uniformly; (S2) adding nitrite into the reactor and stirring for reacting to obtain diazonium salt; (S3) preparing an alkaline solution containing 5-pyrazolone; (S4) adding the alkaline solution containing 5-pyrazolone in the step (S3) into the reactor in the step (S2) for reacting with the diazonium salt to obtain an azo intermediate I; (S5) preparing an alkaline solution of 5-pyrazolone with the same concentration as (S3), and adding into the reactor in the (S4) for reacting with the azo intermediate I to obtain an azo intermediate II; and (S6) after the reaction, performing salting-out, filtration and drying in the reactor to obtain acid yellow N-R powder, wherein R1 is (CH2)nCH3, and n>=0. The method disclosed by the invention has the following beneficial effects: by mainly adopting the benzidine compound as a raw material, the hydrophilic groups contained in the synthesis product acid yellow are reduced, and an effect of improving the washing fastness of acid yellow is realized.