All the comminution stages of iron ore from raw ore size to the concentrate size were taken as one comminution system to research the theoretical relationship between comminution energy and particle size. Firstly, the basic theories of relationship between comminution energy and particle size of ore were analyzed to establish a theoretical model of comminution energy of iron ore. Secondly, the change patterns of comminution energy were analyzed in site investigation, and particle size distribution characteristics in each comminution stage were analyzed in indoor experiments. Lastly, the change patterns of specific energy of the comminution system were analyzed with the method of numerical fitting, and the reliability of the theoretical model of comminution energy was verified with the data of the specific energy, characteristic size, and medium size of the iron ore in each comminution stage. Results showed that the comminution energy of iron ore increased exponentially, the distribution of comminution energy was unreasonable, and the product granularity was characterized by exponential distribution, polynomial function distribution, and Langmuir power function distribution in turn along with the alteration of comminution stage in the system. There was a negative correlation function between the specific energy of the comminution system and the particle size of ore. The change rate of specific energy of the comminution system turned into a turning point when the medium size was 12 mm or the characteristic size was 25 mm. The established theoretical model of comminution energy has larger range of particle size in energy calculation compared with classical models. It can be used to fit the function formulas between specific energy of stage comminution and the size of feed and product and provides theoretical basis for the calculation of comminution energy in iron mine.