In order to solve such problems as difficult development, low grinding grade and high grinding energy consumption of ultra-fine particle embedded poor magnetite, a high-pressure grinding preselection process test is carried out, and the influence of microcracks produced by high-pressure grinding on grinding and separation is studied by comparing the particle size characteristics, relative grindability, Bond ball milling work index and preselection index of two kinds of broken products. The results show that, compared with conventional crushing, the mass fraction of <0.074 mm in rolled products increases by 14.69%, and the mass fraction of coarse particles >2 mm decreases by 8.59%. Under the preselected magnetic field intensity of 160 kA/m, the iron grade of the rolled product concentrate is 31.96%, which is increased by 1.46%. The recovery rate of magnetic iron is as high as 99.45%, increased by 1.17%, and the tail-throwing rate is increased by 5.03%. The test results of relative grindability show that when the grinding fineness reaches less than 0.03 mm and the mass fraction of particle size is 85%, the time consumption of rolling products is shortened by 42.13% compared with conventional crushing. Bond ball milling work index(target particle size <0.15 mm) of rolled products is decreased by 27.90%. The results of polarized light microscope and SEM show that the rolled product has loose and porous structure, fully developed microcracks, a large number of grain boundary cracks and more complete dissociation.