The intermediate o-nitrophenyl diazo salt was synthesized from o-nitroaniline and diazo salt. The compound 1, 2-nitro-4'-dimethylaminoazobenzene was further prepared by coupling diazo salt with N, N-dimethylaniline. The effects of reaction conditions such as pH value, stoichiometric ratio and temperature on the synthesis were investigated. The structure of the compound was characterized by infrared spectroscopy. The acid-base discoloration and spectrum properties of 1 were studied by ultraviolet spectroscopy. The experimental results show that 1 has high molar absorptivity (ε=2.42*10⁴ L/mol.cm) and good acid-base discoloration.