The efficient extraction of iron from nickel slag have attracted more recent attention in the recycling of nickel slag. In this paper, FACTSAGE software was used to predict the formation and preferential precipitation behavior of magnetite crystal in molten nickel oxide slag. The precipitation and growth of magnetite crystal in molten nickel oxide slag during continuous cooling were in-situ observed by high temperature laser confocal microscope (HT-CLSM), and the growth characteristics of magnetite crystal under 5~50 ℃/min cooling rate was studied. The morphology, phase and composition of the samples were characterized by SEM-EDS, XRD and ICP. The results show that magnetite crystals are preferentially precipitated from the melt during continuous cooling, and the initial precipitation temperature is between 1430~1450 ℃. At low cooling rate (5~15 ℃/min), the initial nucleation and precipitation of the crystals are slow, and the grains grow stably at 1200~1400 ℃, with an average growth rate of 0.141 μm/s, the final grain size is more than 100 μm. High cooling rate (25~50 ℃/min) can promote the rapid nucleation and precipitation of magnetite crystal at the initial stage of growth, but the stable growth rate is small, and the final grain size is 20~30 μm.