Abstract: 2,4-dimethyl benzoic acid, an intermediate of the acyl phosphine oxide photoinitiator with best performance was synthesized. The reaction mechanism was studied and sodium hypochlorite was used as the substitution of chlorine generally used in the chloroform reaction. The optimal synthesis conditions were found through orthogonal test as follows：the molar ratio of chlorinated acetophenone to sodium hypochlorite 1∶1.3，the amount of the catalyst (benzalkonium bromide solution) 15 mL，reaction time seven hours，reaction temperature 80 ℃. Repeated test showed that the average highest yield was 80%.