•Diazotization of weakly basic amines occurred in the absence of concentrated sulfuric acid.•Solid diazonium salts with good thermal stability were obtained.•There is no dilution process of concentrated acid in the coupling reaction.•Large amounts of ice are saved and energy consumption can be reduced.A new synthetic strategy for industrially important deep-shade disperse azo dyes was presented in this study. The key procedure is to prepare stable solid diazonium salts of weakly basic amines in the absence of concentrated sulfuric acid. Diazotization by tert-butyl nitrite in ethyl acetate was allowed to proceed in the presence of equivalent 1,5-naphthalenedisulfonic acid as stabilizer of diazonium salts and donor of hydrogen ion for the reaction. The separated solid diazonium salts exhibited good thermal stability. The corresponding disperse azo dyes were subsequently synthesized through the azo-coupling of the prepared solid diazonium salts with a range of aromatic tertiary amines. The azo dyes were produced in short reaction time, excellent yields, mild reaction conditions, simple experimental procedure and low energy consumption.