In order to study the selfsensitizing structure characteristics of ammonium nitrate(AN) used for industrial ammonia dynamite,the pore structure characteristics(such as porosity,pore diameter,specific surface area and pore distribution) and particles shape were investigated by mercury injection method and scanning electric microscopy(SEM). Results show that there are lots of pores and the pore quantity whose size is within the effective hotspots range is much high in 1# expanded ammonium nitrate(1#EAN) sample,its selfsensitizing characteristic is obvious. In 2#EAN sample,there are also many pores,but the pore size is much smaller and the pore distribution is narrow. The 3#EAN sample is similar to the common AN，i.e. the porosity and face area are much less and the pore quantity whose size is within the effective hotspots range is also less.