The invention relates to a high-efficiency method for preparing low-sulfur molybdenum trioxide from molybdenite by roasting, and belongs to the field of molybdenum metallurgy. According to the invention, molybdenite processed through crushing and ball milling is placed in a quartz tube; the quartz tube is placed in a microwave cavity; a protective gas, namely nitrogen gas, is introduced for emptying air in the quartz tube; microwave is turned on, and output power is adjusted; the molybdenite is subjected to microwave radiation pretreatment for 10-20min under nitrogen gas protection; the molybdenite obtained after the microwave pretreatment is placed in a resistance heating furnace; temperature is set to be 550-600 DEG C; compressed air is blown in; the temperature is maintained for 2h, such that molybdenum trioxide with sulfur content lower than 0.1% is obtained. According to the invention, advantages of microwave heating and electronic heating are combined, and the method has the advantages of low energy consumption, high efficiency, high product quality and the like.