The correlations between phase evolution, fusion behavior, and Ni-Fe granules growth of laterite ore–CaO mixtures during reductive roasting have been investigated. The minimum melting point of 1220°C of the CaO-MgO-Al2O3-SiO2 system with 17–36 wt.% CaO is demonstrated via phase diagram analysis, and this point is decreased in the presence of FeO. This reveals that the fusion behavior in close association with the Fe-Ni granular growth can be regulated by altering the contents of CaO and FeO. Promoting the generation of diopside (CaMgSi2O6) may reduce the operating temperature from 1300–1350°C to 1150–1200°C, which ensures sufficient melting phase content. Moreover, reducing the CO partial fraction lowers the fusion temperature but hinders the growth of Ni-Fe grains. The average size of Ni-Fe granules in the reduced mixture with 17 wt.% CaO reaches nearly 20 μm at 1200°C for 1 h in a 100 vol.% CO atmosphere.