DCB diazonium salt aqueous solutions were synthesized from slightly soluble 3,3’-dichlorobenzidine hydrochloride by continuous-flow method via a collision mixer. The concentration of DCB diazonium salt was evaluated by spectrophotometry and the yield of diazotization was calculated. In order to optimize the conditions of diazotization reaction, the effects of DCB concentrations,feed temperature, flow rate, molar ratio of HCl and DCB on the yield of diazotization were investigated and a yield of 98.3% was achieved under the optimized reaction conditions of mass concentration of DCB 70g/L, feed temperature 20℃, flow rate 1.1m/s, n(HCl)∶n(DCB)=4. The continuous process,in comparison with the batch process, requires less acid, improves the reaction efficiency, and can be carried out at room temperature(20℃),which significantly reduces energy consumption and has great potential in industrial applications.