The hardening of MoO 2 not only reduces the yield of subsequent molybdenum powder but also affects the quality of molybdenum powder. To eliminate the hardening of MoO 2 and improve the quality of molybdenum powder, the cause analysis and control method of MoO 2 production in converters and four-tube reduction furnaces was studied. The results show that the hardening of MoO 2 becomes more serious with a lower content of potassium in MoO 3 and a smaller particle size. In addition, the higher the temperature is, the more serious the hardening is. When the flow rate of hydrogen is 40 and 6 m 3 /h, the MoO 2 produced by the rotary converter and flat four-tube furnace is loose. Therefore, MoO 2 hardening can be controlled by simple equipment modification to slow the reaction rate, reduction process optimization and raw material selection.