The important role of sulfur trioxide in caprolactam production by toluene method was anaylzed. The various material properties change of influencing premixed and amidation were analyzed.The main key factors affecting reaction were circular light phase with water and nitroso sulfur trioxide in sulfuric acid, sulfuric acid and the nitroso preparation unit and amidation unit of each unit operation parameters, the phenomenons were analyzed，and combining with laboratory experiments, the corresponding multiple solutions were gradually put forward, so the devices were in the secure full of cycle and stable operation, the consumption data were good, the unit yield and the smooth rate were improved.