Abstract: The present application relates to a method for photochlorination, and specifically to photochlorination by a photochemical reaction of an aromatic compound with gaseous chlorine so as to prepare a trichloromethyl-substituted benzene, and to a method using bis-(trichloromethyl)-benzene as the trichloromethyl-substituted benzene to prepare by further reaction bis-(chloroformyl)-benzene. Through the control of temperature, illuminance and consumption of gaseous chlorine, the method of this application can greatly improve the purity of trichloromethyl-substituted benzene and further prepare polymer-grade bis-(chloroformyl)-benzene with low cost. The present application also relates to a method for purifying trichloromethyl-substituted benzene, and specifically to a method for purifying trichloromethyl-substituted benzene via molecular distillation. The present application further relates to a photochlorination reactor for use in photochlorination reactions (such as those of the present application).