The invention relates to a floatation depressant for obtaining molybdenum concentrate and copper concentrate by mineral separation, particularly by separation of copper-molybdenum mixed concentrate, and methods for preparing and using the floatation depressant. The floatation depressant is characterized by being an organic liquid agent obtained through chemical reaction by using mercaptoacetic acid, gallic acid and sodium hydroxide as raw materials. The floatation depressant is the organic liquid agent obtained through the chemical reaction of pyrogallic acid, the mercaptoacetic acid and the sodium hydroxide, and can be used for effectively depressing primary and secondary copper sulfide ores such as copper pyrite and chalcocite, and the copper-molybdenum mixed concentrate can be effectively separated. The defects of large usage of common sodium sulfide, severe operation environment and serious pollution are overcome, and the floatation depressant has the advantages of convenience in addition, safety in use and the like. An effective method which is small in agent usage, convenient to use and low in environmental pollution is provided for the separation of molybdenum copper sulfide mixed concentrate.