Abstract: In this study, the mineral composition and embedded features of the nickel slag as well as its phase transformation in a settlement furnace during deep reduction were investigated through chemical composition analysis, X-ray diffraction, optical microscopy, scanning electron microscopy, and energy dispersive spectrometry. The results show that the phase composition of slag includes hortonolite and glass. The Cu-Ni-Fe sulfide mineral is distributed in the silicate irregularly, and the sulfide material is too small to reclaim. The nickel slag transforms into akermanite, ferronickel, augite, cancrinite, riebeckite, and quartz by deep reduction. The process is characterized by constant mineral components of reduzate, and the contents of akermanite and ferronickel are the most when heated to 1300℃. The reduction time is also an important factor during the process, and the contents of ferronickel increase over time, maximizing at 120 min. The thermodynamic analysis shows that the main reaction performed during the reduction is that olivine and calcium oxide transform into akermanite and FeO, and then the FeO is reduced to iron by C and CO. The metal sulfides, calcium oxide, and C are transformed into copper and nickel and then dissolved in the iron, and the CaS then crystallized out with the silicate minerals.