Aiming at the problems of the large increase of weight and thickness of shell steel for ammonia synthesis tower, as well as the increasingly stringent technical requirements, the extrathick electroslag plate for the shell of large ammonia synthesis tower was successfully developed in Wugang based on the safe, stable and efficient design of innovative components for the key chemical equipment operation and combined with the equipment level of current production line by creatively adopting electroslag production process. The results showed that the steel was pure, whose composition was homogeneous and structure was compact and fine, and had low carbon equivalent, excellent tensile properties at room temperature, high temperature and good impact properties at low temperature. The internal and external quality of steel was excellent. The thickness of the steel plate was 177 mm, the weight of the finished product was 37.9 t. The thickness and weight of steel plate were the only one in China. The technical requirement was close to the limit level of this kind of steel at the present stage. The steel plate was used for the design and manufacture of the large ammonia converter in a domestic chemical project instead of the imported steel plate.