A type of large-grained magnesium olivine was used as Mg additive to prepare pellet. Effect of olivine particle size distribution was studied by changing pre-grinding time of olivine. Properties of raw materials and prepared pellets were analyzed by Chemical analysis, XRD, SEM-EDS, etc. Results showed that, the pre-grinding time of the magnesium olivine is beneficial to improving the performance of green pellet, there are two kinds of pores in a roasted pellet, the circular large pores caused by liquid phase generation and hematite intergranular pores, both of which have a certain effect on the strength of pellet. With the increase of the pre-grinding time, the shape and distribution of circular large pores in pellets gradually become uniform, pore size and quantity of hematite intergranular pores become smaller, compressive strength of the pellet increased at first and then decreased slowly, porosity changes in the opposite direction. The optimal pre-grinding time of magnesium olivine is 40 min, under which condition can the olivine and pellet meet the requirements of production practice.