In order to understand the microstructures of ammonium nitrate-oil explosives(ANFO) and their effects on detonator initiation sensitivity, different types of ANFO were made from fuel oil and four different states AN, and the scanning electric microscopy (SEM) and density measuring method were both used to characterize their microstructures. Then, the section technique was used for the first time to observe their intragranular microstructures, and plate dent test was conducted to evaluate the detonator initiation sensitivity. Results show that unexpanded ANFO particles have few pores and cracks inside and outside of the particles, and its surface is more smoother than that of expanded ANFO particles. While expanded ANFO particles have many holes and cracks, there are abundant voids and twin crystals lying in the interior of expanded ANFO particles, and the growth of crystals is incomplete. The microstructures of porous particles and protruding particle surface are helpful to enhance the detonator initiation sensitivity of ANFO.