4,6-Diaminoresorcinol which is the monomer of PBO fiber was synthesized with 2-chloro-4,6-dinitroresorcinol as an important intermediate.It was prepared with 1,2,3- trichlorobenzene as the start material  through the nitration process and basic hydrolysis process. The optimized reaction conditions were that in nitration, ratio of 1,2,3-trichlorobenzene to nitric acid was 1∶2.24,reaction temperature was 65 ℃,reaction time was 5 h,in basic hydrolysis reaction, ratio of 2-chloro-4,6-dinitroresorcinol to sodium hydroxide was 1∶6.5,reaction temperature was 75 ℃, reaction time was 8 h. The yield of 2-chloro-4,6-dinitroresorcinol was 90.4% under the optimal conditions.